Tree-sitter

[kubukoz]

TODOs:

• extract e2e test change to its own PR
• figure out what to do about ASTAdapter and the "old" AST in general
• ??? in resolveServiceTs
• ??? in resolveExplicitTs
• unify Ts variants with non-Ts variants in MultiServiceResolver (maybe decode everything with the adapter before even calling MSR? or have a helper that does that in one go?
• some tests for completions in broken files of all kinds
• parser-gen: add some association of SourceFile and smithyql? (grammar.json has the language name)