feat: add raw byte access methods and \x escape to String class

Add `byteAt(index)`, `codePointAt(index)`, and `bytes` sequence to String,
enabling direct byte-level manipulation of UTF-8 encoded strings. Introduce
`\x` hex escape sequence in string literals for specifying raw byte values.
Refactor Unicode escape parsing into a generic `readHexEscape` function
supporting variable digit counts. Implement `wrenUtf8Decode` utility for
decoding UTF-8 sequences from byte buffers. Add comprehensive tests for
`byteAt` including boundary conditions and error cases.

builtin/core.wren

@@ -92,7 +92,19 @@ class Sequence {
   }
 }
 
-class String is Sequence {}
+class String is Sequence {
+  bytes { new StringByteSequence(this) }
+}
+
+class StringByteSequence is Sequence {
+  new(string) {
+    _string = string
+  }
+
+  [index] { _string.byteAt(index) }
+  iterate(iterator) { _string.iterateByte_(iterator) }
+  iteratorValue(iterator) { _string.byteAt(iterator) }
+}
 
 class List is Sequence {
   addAll(other) {