feat: add hexadecimal literal support to lexer and compiler

Add TOKEN_HEXADECIMAL token type and isHexDigit helper function to lexer.
Modify readNumber to detect 'x' prefix after '0' and lex hex digits.
Implement emitNumericConstant for hex values in compiler.
Add test file test/number/hex_literals.wren verifying basic hex parsing,
arithmetic, type checks, and negative hex literals.

test/number/hex_literals.wren

@@ -0,0 +1,8 @@
+var x = 0xFF
+
+IO.print(x) // expect: 255
+IO.print(x + 1) // expect: 256
+IO.print(x == 255) // expect: true
+IO.print(0x09 is Num) // expect: true
+IO.print(x is Num) // expect: true
+IO.print(-0xFF) // expect: -255