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_sequence/iterate.wren

@@ -0,0 +1,21 @@
+// Bytes:
+//        012345678
+// Chars: sø mé ஃ
+var bytes = "søméஃ".bytes
+
+IO.print(bytes.iterate(null)) // expect: 0
+IO.print("".bytes.iterate(null)) // expect: false
+
+IO.print(bytes.iterate(0)) // expect: 1
+IO.print(bytes.iterate(1)) // expect: 2
+IO.print(bytes.iterate(2)) // expect: 3
+IO.print(bytes.iterate(3)) // expect: 4
+IO.print(bytes.iterate(4)) // expect: 5
+IO.print(bytes.iterate(5)) // expect: 6
+IO.print(bytes.iterate(6)) // expect: 7
+IO.print(bytes.iterate(7)) // expect: 8
+IO.print(bytes.iterate(8)) // expect: false
+
+// Out of bounds.
+IO.print(bytes.iterate(123)) // expect: false
+IO.print(bytes.iterate(-1)) // expect: false

test/core/string_byte_sequence/iterate_not_int.wren

@@ -0,0 +1 @@
+"str".bytes.iterate(12.34) // expect runtime error: Iterator must be an integer.

test/core/string_byte_sequence/iterate_wrong_type.wren

@@ -0,0 +1 @@
+"str".bytes.iterate("not num") // expect runtime error: Iterator must be a number.

test/core/string_byte_sequence/iterator_value.wren

@@ -0,0 +1,24 @@
+// Bytes:
+//        012345678
+// Chars: sø mé ஃ
+var bytes = "søméஃ".bytes
+
+IO.print(bytes.iteratorValue(0)) // expect: 115
+IO.print(bytes.iteratorValue(1)) // expect: 195
+IO.print(bytes.iteratorValue(2)) // expect: 184
+IO.print(bytes.iteratorValue(3)) // expect: 109
+IO.print(bytes.iteratorValue(4)) // expect: 195
+IO.print(bytes.iteratorValue(5)) // expect: 169
+IO.print(bytes.iteratorValue(6)) // expect: 224
+IO.print(bytes.iteratorValue(7)) // expect: 174
+IO.print(bytes.iteratorValue(8)) // expect: 131
+
+IO.print(bytes.iteratorValue(-9)) // expect: 115
+IO.print(bytes.iteratorValue(-8)) // expect: 195
+IO.print(bytes.iteratorValue(-7)) // expect: 184
+IO.print(bytes.iteratorValue(-6)) // expect: 109
+IO.print(bytes.iteratorValue(-5)) // expect: 195
+IO.print(bytes.iteratorValue(-4)) // expect: 169
+IO.print(bytes.iteratorValue(-3)) // expect: 224
+IO.print(bytes.iteratorValue(-2)) // expect: 174
+IO.print(bytes.iteratorValue(-1)) // expect: 131

test/core/string_byte_sequence/iterator_value_not_int.wren

@@ -0,0 +1 @@
+"abcd".bytes.iteratorValue(12.34) // expect runtime error: Index must be an integer.

test/core/string_byte_sequence/iterator_value_not_num.wren

@@ -0,0 +1 @@
+"abcd".bytes.iteratorValue("not num") // expect runtime error: Index must be a number.

test/core/string_byte_sequence/iterator_value_too_large.wren

@@ -0,0 +1 @@
+"abcd".bytes.iteratorValue(4) // expect runtime error: Index out of bounds.

test/core/string_byte_sequence/iterator_value_too_small.wren

@@ -0,0 +1 @@
+"abcd".bytes.iteratorValue(-5) // expect runtime error: Index out of bounds.

test/core/string_byte_sequence/subscript.wren

@@ -0,0 +1,24 @@
+// Bytes:
+//        012345678
+// Chars: sø mé ஃ
+var bytes = "søméஃ".bytes
+
+IO.print(bytes[0]) // expect: 115
+IO.print(bytes[1]) // expect: 195
+IO.print(bytes[2]) // expect: 184
+IO.print(bytes[3]) // expect: 109
+IO.print(bytes[4]) // expect: 195
+IO.print(bytes[5]) // expect: 169
+IO.print(bytes[6]) // expect: 224
+IO.print(bytes[7]) // expect: 174
+IO.print(bytes[8]) // expect: 131
+
+IO.print(bytes[-9]) // expect: 115
+IO.print(bytes[-8]) // expect: 195
+IO.print(bytes[-7]) // expect: 184
+IO.print(bytes[-6]) // expect: 109
+IO.print(bytes[-5]) // expect: 195
+IO.print(bytes[-4]) // expect: 169
+IO.print(bytes[-3]) // expect: 224
+IO.print(bytes[-2]) // expect: 174
+IO.print(bytes[-1]) // expect: 131

test/core/string_byte_sequence/subscript_not_int.wren

@@ -0,0 +1 @@
+"abcd".bytes[12.34] // expect runtime error: Index must be an integer.

test/core/string_byte_sequence/subscript_not_num.wren

@@ -0,0 +1 @@
+"abcd".bytes["not num"] // expect runtime error: Index must be a number.

test/core/string_byte_sequence/subscript_too_large.wren

@@ -0,0 +1 @@
+"abcd".bytes[4] // expect runtime error: Index out of bounds.

test/core/string_byte_sequence/subscript_too_small.wren

@@ -0,0 +1 @@
+"abcd".bytes[-5] // expect runtime error: Index out of bounds.