Search by unicode scalar when in that mode

Previously, searching via firstMatch or matches(of:) would only
_start_ searches at a character index, even when a regex has
Unicode scalar semantics.

Author: natecook1000

Sources/_StringProcessing/Regex/Match.swift

@@ -158,7 +158,11 @@ extension Regex {
       if let m = try _match(input, in: low..<high, mode: .partialFromFront) {
         return m
       }
-      input.formIndex(after: &low)
+      if regex.program.loweredProgram.initialOptions.semanticLevel == .graphemeCluster {
+        input.formIndex(after: &low)
+      } else {
+        input.unicodeScalars.formIndex(after: &low)
+      }
     }
     return nil
   }

Tests/RegexTests/MatchTests.swift

@@ -1612,5 +1612,15 @@ extension RegexTests {
 
   // TODO: Add test for grapheme boundaries at start/end of match
 
+  func testCase() {
+    let regex = try! Regex(#".\N{SPARKLING HEART}."#)
+    let input = "🧟‍♀️💖🧠 or 🧠💖☕️"
+    let characterMatches = input.matches(of: regex)
+    XCTAssertEqual(characterMatches.map { $0.0 }, ["🧟‍♀️💖🧠", "🧠💖☕️"])
+
+    let scalarMatches = input.matches(of: regex.matchingSemantics(.unicodeScalar))
+    let scalarExpected: [Substring] = ["\u{FE0F}💖🧠", "🧠💖☕"]
+    XCTAssertEqual(scalarMatches.map { $0.0 }, scalarExpected)
+  }
 }