[ty] Implement patterns and typevars in the LSP (#21671)
## Summary
**This is the final goto-targets with missing
goto-definition/declaration implementations!
You can now theoretically click on all the user-defined names in all the
syntax. 🎉**
This adds:
* goto definition/declaration on patterns/typevars
* find-references/rename on patterns/typevars
* fixes syntax highlighting of `*rest` patterns
This notably *does not* add:
* goto-type for patterns/typevars
* hover for patterns/typevars (because that's just goto-type for names)
Also I realized we were at the precipice of one of the great GotoTarget
sins being resolved, and so I made import aliases also resolve to a
ResolvedDefinition. This removes a ton of cruft and prevents further
backsliding.
Note however that import aliases are, in general, completely jacked up
when it comes to find-references/renames (both before and after this
PR). Previously you could try to rename an import alias and it just
wouldn't do anything. With this change we instead refuse to even let you
try to rename it.
Sorting out why import aliases are jacked up is an ongoing thing I hope
to handle in a followup.
## Test Plan
You'll surely not regret checking in 86 snapshot tests
