test: verify all keywords of grammar are available in completions

[juleswritescode]

Adds two tests that make sure:

1. new keywords added to grammar.js are included in the keyword-providers
2. all keywords in keyword-providers are included in grammar.js

I tried generating the tests from the pg_query.proto first, but some tokens such as "btree" and "gin" are no "official postgres keywords" — we do want to suggest them as keywords at the right spots, though.

Regarding keyword generation: I think generating the keywords from either protobuf or treesitter-grammar doesn't really make sense: We have the .starts_statement() and .require_prefix() builder pattern, and I figure there'll be more of those options soon; so we'd need some manual code anyways.

[juleswritescode]

these keywords were unreachable 👍🏻

[psteinroe]

the KW list from postgres includes a keyword type to differentiate btw reserved and non-reserved keywords. did u check if the list we want here could be generated from that?

[juleswritescode]

the KW list from postgres includes a keyword type to differentiate btw reserved and non-reserved keywords. did u check if the list we want here could be generated from that?

Yeah I did check it, but the issue is that we have some completable "keywords" that aren't even tokens in the Postgres protobuf.

For example, for create index my_cool_idx on auth.users using |, we'd want to suggest btree, hash, gist, spgist, gin, or brin. These aren't even listed in the protobuf file.

So our "keyword completion" is actually more a "valid token at this position" completion.