- Made `NativeApi` into a `static class` (it's not intended to be instantiated)
- Moved `LLamaTokenType` enum out into a separate file
- Made `LLamaSeqId` and `LLamaPos` into `record struct`, convenient to have equality etc
- Made all the mechanics of grammar parsing (GBNFGrammarParser, ParseState) internal. Just call `Grammar.Parse("whatever")`.
- Added a `GrammarRule` class which validates elements on construction (this allows constructing grammar without parsing GBNF).
- It should be impossible for a `GrammarRule` to represent an invalid rule.