[Refactor] Refactor GrammarMatcherBase

[Ubospica]

This PR refactors the GrammarMatcherBase for simpler logic. It mainly updates the ExpandEquivalentStackElements (renamed from ExpandStackElement) function: it elegantly utilizes recursion to make it a lot simpler than the previous version.

This PR also fixes a bug in parser which could lead to incorrect parsing for character class stars. Check out the test.

[Ubospica]

Performance:

pytest /home/yixind/xgrammar/tests/python/test_grammar_matcher_json.py::test_json_pressure
pytest /home/yixind/xgrammar/tests/python/test_grammar_matcher_json.py::test_fill_next_token_bitmask

• Json pressure:
  • Prev: 328.732 us, 964.586 us
  • Now: 355.181 us, 883.275 us
• Compile Grammar:
  • Prev:

    Time to init GrammarMatcher: 25143.588 us
    Time to init GrammarMatcher: 78032.158 us
    Time to init GrammarMatcher: 24902.84 us
    

  • Now:

    Time to init GrammarMatcher: 25587.555 us
    Time to init GrammarMatcher: 74758.774 us
    Time to init GrammarMatcher: 24317.109 us