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/core/string/byte_at.wren

@@ -0,0 +1,38 @@
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+var s = "søméஃthîng"
+
+IO.print(s.byteAt(0)) // expect: 115
+IO.print(s.byteAt(1)) // expect: 195
+IO.print(s.byteAt(2)) // expect: 184
+IO.print(s.byteAt(3)) // expect: 109
+IO.print(s.byteAt(4)) // expect: 195
+IO.print(s.byteAt(5)) // expect: 169
+IO.print(s.byteAt(6)) // expect: 224
+IO.print(s.byteAt(7)) // expect: 174
+IO.print(s.byteAt(8)) // expect: 131
+IO.print(s.byteAt(9)) // expect: 116
+IO.print(s.byteAt(10)) // expect: 104
+IO.print(s.byteAt(11)) // expect: 195
+IO.print(s.byteAt(12)) // expect: 174
+IO.print(s.byteAt(13)) // expect: 110
+IO.print(s.byteAt(14)) // expect: 103
+
+IO.print(s.byteAt(-15)) // expect: 115
+IO.print(s.byteAt(-14)) // expect: 195
+IO.print(s.byteAt(-13)) // expect: 184
+IO.print(s.byteAt(-12)) // expect: 109
+IO.print(s.byteAt(-11)) // expect: 195
+IO.print(s.byteAt(-10)) // expect: 169
+IO.print(s.byteAt(-9)) // expect: 224
+IO.print(s.byteAt(-8)) // expect: 174
+IO.print(s.byteAt(-7)) // expect: 131
+IO.print(s.byteAt(-6)) // expect: 116
+IO.print(s.byteAt(-5)) // expect: 104
+IO.print(s.byteAt(-4)) // expect: 195
+IO.print(s.byteAt(-3)) // expect: 174
+IO.print(s.byteAt(-2)) // expect: 110
+IO.print(s.byteAt(-1)) // expect: 103
+
+IO.print("\0".byteAt(0)) // expect: 0

test/core/string/byte_at_not_int.wren

@@ -0,0 +1 @@
+IO.print("string".byteAt(12.34)) // expect runtime error: Index must be an integer.

test/core/string/byte_at_not_num.wren

@@ -0,0 +1 @@
+IO.print("string".byteAt("not num")) // expect runtime error: Index must be a number.

test/core/string/byte_at_too_large.wren

@@ -0,0 +1 @@
+IO.print("string".byteAt(6)) // expect runtime error: Index out of bounds.

test/core/string/byte_at_too_small.wren

@@ -0,0 +1 @@
+IO.print("string".byteAt(-7)) // expect runtime error: Index out of bounds.

test/core/string/bytes.wren

@@ -0,0 +1,6 @@
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+var s = "søméஃthîng"
+
+IO.print(s.bytes is StringByteSequence) // expect: true

test/core/string/code_point_at.wren

@@ -0,0 +1,38 @@
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+var s = "søméஃthîng"
+
+IO.print(s.codePointAt(0)) // expect: 115
+IO.print(s.codePointAt(1)) // expect: 248
+IO.print(s.codePointAt(2)) // expect: -1
+IO.print(s.codePointAt(3)) // expect: 109
+IO.print(s.codePointAt(4)) // expect: 233
+IO.print(s.codePointAt(5)) // expect: -1
+IO.print(s.codePointAt(6)) // expect: 2947
+IO.print(s.codePointAt(7)) // expect: -1
+IO.print(s.codePointAt(8)) // expect: -1
+IO.print(s.codePointAt(9)) // expect: 116
+IO.print(s.codePointAt(10)) // expect: 104
+IO.print(s.codePointAt(11)) // expect: 238
+IO.print(s.codePointAt(12)) // expect: -1
+IO.print(s.codePointAt(13)) // expect: 110
+IO.print(s.codePointAt(14)) // expect: 103
+
+IO.print(s.codePointAt(-15)) // expect: 115
+IO.print(s.codePointAt(-14)) // expect: 248
+IO.print(s.codePointAt(-13)) // expect: -1
+IO.print(s.codePointAt(-12)) // expect: 109
+IO.print(s.codePointAt(-11)) // expect: 233
+IO.print(s.codePointAt(-10)) // expect: -1
+IO.print(s.codePointAt(-9)) // expect: 2947
+IO.print(s.codePointAt(-8)) // expect: -1
+IO.print(s.codePointAt(-7)) // expect: -1
+IO.print(s.codePointAt(-6)) // expect: 116
+IO.print(s.codePointAt(-5)) // expect: 104
+IO.print(s.codePointAt(-4)) // expect: 238
+IO.print(s.codePointAt(-3)) // expect: -1
+IO.print(s.codePointAt(-2)) // expect: 110
+IO.print(s.codePointAt(-1)) // expect: 103
+
+IO.print("\0".codePointAt(0)) // expect: 0

test/core/string/code_point_at_incomplete.wren

@@ -0,0 +1,3 @@
+// The first two bytes of a three-octet sequence.
+var s = "\xe0\xae"
+IO.print(s.codePointAt(0)) // expect: -1

test/core/string/code_point_at_not_int.wren

@@ -0,0 +1 @@
+IO.print("string".codePointAt(12.34)) // expect runtime error: Index must be an integer.

test/core/string/code_point_at_not_num.wren

@@ -0,0 +1 @@
+IO.print("string".codePointAt("not num")) // expect runtime error: Index must be a number.

test/core/string/code_point_at_too_large.wren

@@ -0,0 +1 @@
+IO.print("string".codePointAt(6)) // expect runtime error: Index out of bounds.

test/core/string/code_point_at_too_small.wren

@@ -0,0 +1 @@
+IO.print("string".codePointAt(-7)) // expect runtime error: Index out of bounds.