Implement hexadecimal number literal support in the Wren compiler by adding a `number` field to the Parser struct, a `readHexDigit` helper that returns -1 for invalid hex chars, and a `readHexNumber` function that skips the 'x' prefix, accumulates hex digits, converts via strtol, and emits a TOKEN_NUMBER. Update `readNumber` to remove the TODO placeholder for hex support. Include test cases for valid hex literals (0xFF, 0x09, negative -0xFF) and an invalid hex literal (2xFF) that triggers a lex error.