feat: add hexadecimal number literal parsing to wren compiler

Remove the TOKEN_HEXADECIMAL token type and isHexDigit helper, replacing them with a readHexDigit function and readHexNumber parser that directly converts hex literals to numeric values during lexing. Add a `number` field to the Parser struct to hold the parsed value, and include a test for invalid hex literal error handling.

test/number/invalid_hex_literal.wren

@@ -0,0 +1 @@
+var x = 2xFF // expect error: Error at 'xFF': Expect end of file.