Support contextual top-level snippets

[crtschin]

Closes #4905.

Ended up going a bit in the rabbit hole. This adds top-level snippets and includes a bit better nested behavior in deducing the context of completion.

• It won't suggest snippets that aren't valid in the current context. So no import snippets if declarations started. No declaration snippets prior any import.
• The context that's returned is the tightest one around the completion requests' range.
• I tried to minimize the amount of the AST that's traversed with syb.

[crtschin]

So there's this funny thing, if the context ranges are too tight, then when typing it's possible to fall out of the edge of a range as the contexts won't follow invalid parses.

This causes a lot of the existing tests to fail, as they put the cursor beyond the range of text. To partially get around this, I don't compare columns when determining whether a context range applies to the current range. This gets it to at pass the existing tests, without me having to manually correct the positions at each one.

[crtschin]

This PR is a bit much and does multiple things at the same time. If anyone is looking at this, I'm going to split it up so it's easier to review.

[fendor]

@crtschin I asked @VeryMilkyJoe to review this PR, please ping them once you've split it up :) Otherwise, they'll look at this PR in the future.

[crtschin]

I've split this into #4969, which contains the nested context fixes, and a TBD follow-up that'll add the actual snippets.

Please review that PR instead @VeryMilkyJoe whenever you have some time!