feat: implement UTF-8 aware string subscript ranges with new wrenNewStringFromRange helper

Add wrenNewStringFromRange function to wren_value.c that decodes UTF-8 code points from source string and re-encodes them into result, enabling correct range subscripts on multi-byte characters. Rename wrenUtf8NumBytes to wrenUtf8EncodeNumBytes and make wrenUtf8Encode return written byte count for use in range construction. Add wrenUtf8DecodeNumBytes utility to skip continuation bytes when iterating by byte index. Remove all skip directives from test files and add UTF-8 specific test cases verifying correct slicing across code point boundaries.

test/core/string/subscript_range.wren

@@ -1,4 +1,3 @@
-// skip: Range subscripts for strings don't handle UTF-8.
 var string = "abcde"
 IO.print(string[0..0]) // expect: a
 IO.print(string[1...1] == "") // expect: true
@@ -33,3 +32,16 @@ IO.print(string[3...-6]) // expect: dcba
 // An empty range at zero is allowed on an empty string.
 IO.print(""[0...0] == "") // expect: true
 IO.print(""[0..-1] == "") // expect: true
+
+// Indexes by byte, not code point.
+//
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+IO.print("søméஃthîng"[0..3]) // expect: søm
+IO.print("søméஃthîng"[3...10]) // expect: méஃt
+
+// Only includes sequences whose first byte is in the range.
+IO.print("søméஃthîng"[2..6]) // expect: méஃ
+IO.print("søméஃthîng"[2...6]) // expect: mé
+IO.print("søméஃthîng"[2...7]) // expect: méஃ