Use OSC 52 as a fallback for setting the system clipboard

This adds a simple base64 implementation to keep us from adding a crate for one function. It's
mostly based on
https://github.com/marshallpierce/rust-base64/blob/a675443d327e175f735a37f574de803d6a332591/src/engine/naive.rs#L42

helix-view/src/base64/LICENSE-MIT

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+The MIT License (MIT)
+
+Copyright (c) 2015 Alice Maz
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

helix-view/src/base64/README.md

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+A minimal base64 implementation to keep from pulling in a crate for just that. It's based on 
+https://github.com/marshallpierce/rust-base64 but without all the customization options.
+The biggest portion comes from
+https://github.com/marshallpierce/rust-base64/blob/a675443d327e175f735a37f574de803d6a332591/src/engine/naive.rs#L42
+Thanks, rust-base64!

helix-view/src/base64/mod.rs

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+use std::ops::{BitAnd, BitOr, Shl, Shr};
+
+const PAD_BYTE: u8 = b'=';
+const ENCODE_TABLE: &[u8] =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".as_bytes();
+const LOW_SIX_BITS: u32 = 0x3F;
+
+pub fn encode(input: &[u8]) -> String {
+    let rem = input.len() % 3;
+    let complete_chunks = input.len() / 3;
+    let remainder_chunk = if rem == 0 { 0 } else { 1 };
+    let encoded_size = (complete_chunks + remainder_chunk) * 4;
+
+    let mut output = vec![0; encoded_size];
+
+    // complete chunks first
+    let complete_chunk_len = input.len() - rem;
+
+    let mut input_index = 0_usize;
+    let mut output_index = 0_usize;
+    while input_index < complete_chunk_len {
+        let chunk = &input[input_index..input_index + 3];
+
+        // populate low 24 bits from 3 bytes
+        let chunk_int: u32 =
+            (chunk[0] as u32).shl(16) | (chunk[1] as u32).shl(8) | (chunk[2] as u32);
+        // encode 4x 6-bit output bytes
+        output[output_index] = ENCODE_TABLE[chunk_int.shr(18) as usize];
+        output[output_index + 1] = ENCODE_TABLE[chunk_int.shr(12_u8).bitand(LOW_SIX_BITS) as usize];
+        output[output_index + 2] = ENCODE_TABLE[chunk_int.shr(6_u8).bitand(LOW_SIX_BITS) as usize];
+        output[output_index + 3] = ENCODE_TABLE[chunk_int.bitand(LOW_SIX_BITS) as usize];
+
+        input_index += 3;
+        output_index += 4;
+    }
+
+    // then leftovers
+    if rem == 2 {
+        let chunk = &input[input_index..input_index + 2];
+
+        // high six bits of chunk[0]
+        output[output_index] = ENCODE_TABLE[chunk[0].shr(2) as usize];
+        // bottom 2 bits of [0], high 4 bits of [1]
+        output[output_index + 1] = ENCODE_TABLE
+            [(chunk[0].shl(4_u8).bitor(chunk[1].shr(4_u8)) as u32).bitand(LOW_SIX_BITS) as usize];
+        // bottom 4 bits of [1], with the 2 bottom bits as zero
+        output[output_index + 2] =
+            ENCODE_TABLE[(chunk[1].shl(2_u8) as u32).bitand(LOW_SIX_BITS) as usize];
+        output[output_index + 3] = PAD_BYTE;
+    } else if rem == 1 {
+        let byte = input[input_index];
+        output[output_index] = ENCODE_TABLE[byte.shr(2) as usize];
+        output[output_index + 1] =
+            ENCODE_TABLE[(byte.shl(4_u8) as u32).bitand(LOW_SIX_BITS) as usize];
+        output[output_index + 2] = PAD_BYTE;
+        output[output_index + 3] = PAD_BYTE;
+    }
+    String::from_utf8(output).expect("Invalid UTF8")
+}
+
+#[cfg(test)]
+mod tests {
+    fn compare_encode(expected: &str, target: &[u8]) {
+        assert_eq!(expected, super::encode(target));
+    }
+
+    #[test]
+    fn encode_rfc4648_0() {
+        compare_encode("", b"");
+    }
+
+    #[test]
+    fn encode_rfc4648_1() {
+        compare_encode("Zg==", b"f");
+    }
+
+    #[test]
+    fn encode_rfc4648_2() {
+        compare_encode("Zm8=", b"fo");
+    }
+
+    #[test]
+    fn encode_rfc4648_3() {
+        compare_encode("Zm9v", b"foo");
+    }
+
+    #[test]
+    fn encode_rfc4648_4() {
+        compare_encode("Zm9vYg==", b"foob");
+    }
+
+    #[test]
+    fn encode_rfc4648_5() {
+        compare_encode("Zm9vYmE=", b"fooba");
+    }
+
+    #[test]
+    fn encode_rfc4648_6() {
+        compare_encode("Zm9vYmFy", b"foobar");
+    }
+
+    #[test]
+    fn encode_all_ascii() {
+        let mut ascii = Vec::<u8>::with_capacity(128);
+
+        for i in 0..128 {
+            ascii.push(i);
+        }
+
+        compare_encode(
+            "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7P\
+         D0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn8\
+         =",
+            &ascii,
+        );
+    }
+
+    #[test]
+    fn encode_all_bytes() {
+        let mut bytes = Vec::<u8>::with_capacity(256);
+
+        for i in 0..255 {
+            bytes.push(i);
+        }
+        bytes.push(255); //bug with "overflowing" ranges?
+
+        compare_encode(
+            "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7P\
+         D0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn\
+         +AgYKDhIWGh4iJiouMjY6PkJGSk5SVlpeYmZqbnJ2en6ChoqOkpaanqKmqq6ytrq+wsbKztLW2t7i5uru8vb6\
+         /wMHCw8TFxsfIycrLzM3Oz9DR0tPU1dbX2Nna29zd3t/g4eLj5OXm5+jp6uvs7e7v8PHy8/T19vf4+fr7/P3+/w==",
+            &bytes,
+        );
+    }
+}