chore: add xtask to download regression tests

[psteinroe]

EDIT: Removed the tests - this PR is just the xtask now :) can't tag you as a reviewer for some reason, but this is ready @sbdchd

adds the postgress regression test suite to stress-test the parser:

• a xtask that downloads and preprocesses the sql files from the postgres repo
• a test in the squawk_parser crate that runs all of them through the parser

In the preprocessing step, we remove everything that we do not want to support (yet), mainly plpgsql commands such as \gset.

Right now, all test fail. I peeked through some of the tests and it's mainly due to :name vars. I think it makes sense to lex them as IDENT tokens? Happy to add support for this.

Overall I think this test suite can help stabilise the parser. My hope is that it becomes stable enough to replace libpg_query in the postgres language server.

Also: not sure where to put the tests since currently they all are part of the crate itself. For this kind of tests, I usually integrate them as integration tests. Let me know what you prefer! :)

also happy to not add them at all, or maybe not do snapshot testing, or maybe download the files and run the tests on-demand to not pollute the codebase! maybe a separate xtask that can be executed on-demand in CI / locally makes sense too - like https://areweturboyet.com