Replace stdlib calls: isxdigit -> iswxdigit, wcscmp -> memcmp

[maxbrunsfeld]

This PR change the external scanner to use the standard set of libc functions defined in tree-sitter/tree-sitter#2897, which will make it possible to consume these parsers as WASM files.

Checklist

• All tests pass in CI
• There are enough tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (x rules renamed)
• The conflicts section hasn't grown too much (x new conflicts)
• The parser size hasn't grown too much (master: STATE_COUNT, PR: STATE_COUNT)
  (check the value of STATE_COUNT in src/parser.c)

[amaanq]

Nice, thanks!