Add the QUALIFY keyword

[ggevay]

This PR adds the QUALIFY keyword, which is a new reserved keyword. We should quickly add this now, before the first self-managed release is cut, because now we still can check that no user would get in conflict with the new reserved keyword by checking all cloud usage.

Discussed on Slack here and here.

I'm running a Snowflake query to check for occurrences right now, because of the reservedness of the keyword, see Slack discussions. (I checked it once 3 months ago, but checking again now.) Edit: Done. The new Snowflake query also didn't turn up any conflicts for either QUALIFY or WINDOW.

Motivation

• This PR adds a known-desirable feature: https://github.com/MaterializeInc/database-issues/issues/6309

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[jkosh44]

LGTM, but can you add some parser tests?

[ggevay]

Thanks, will add parser tests!