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.

test/language/string/byte_escapes.wren

@@ -0,0 +1,12 @@
+var s = "\x00\x12\x34\x56\x78\xab\xCD\xfFf"
+
+IO.print(s.byteAt(0)) // expect: 0
+IO.print(s.byteAt(1)) // expect: 18
+IO.print(s.byteAt(2)) // expect: 52
+IO.print(s.byteAt(3)) // expect: 86
+IO.print(s.byteAt(4)) // expect: 120
+IO.print(s.byteAt(5)) // expect: 171
+IO.print(s.byteAt(6)) // expect: 205
+IO.print(s.byteAt(7)) // expect: 255
+// "f".
+IO.print(s.byteAt(8)) // expect: 102

test/language/string/incomplete_byte_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0"

test/language/string/incomplete_byte_escape_at_eof.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0

test/language/string/invalid_byte_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0!"