fix(hstr): Fix unsoundness of wtf8's transmutation (#11194)

**Description:**

This PR fixed an unsoundness of `wtf8`'s transmutation by adding `#[repr(transparent)]` to `struct Wtf8`.

The original forked code didn't add this requirement, making the using of `std::mem::transmute` unsound.

Usage example: https://github.com/h-a-n-a/swc/blob/7142def3e26ea9607c0787714b414c31c2872117/crates/hstr/src/wtf8/mod.rs#L462-L464

Author: h-a-n-a

.changeset/light-rats-peel.md

@@ -0,0 +1,6 @@
+---
+swc_core: patch
+hstr: patch
+---
+
+fix(hstr): Fix unsoundness of `wtf8`'s transmutation

crates/hstr/src/wtf8/mod.rs

@@ -434,7 +434,8 @@ impl Extend<CodePoint> for Wtf8Buf {
 /// A borrowed slice of well-formed WTF-8 data.
 ///
 /// Similar to `&str`, but can additionally contain surrogate code points
-/// if they’re not in a surrogate pair.
+/// if they're not in a surrogate pair.
+#[repr(transparent)]
 pub struct Wtf8 {
     bytes: [u8],
 }