Add hover support for in keyword (#1926)

[jackulau]

Summary

• Adds contextual hover for the in keyword in membership tests (shows __contains__ method signature)
• Adds contextual hover for the in keyword in iteration contexts (shows iterable type)

Fixes #1926

Test plan

• Added tests for in in membership test with custom __contains__
• Added tests for in in for-loops
• Added tests for in in list comprehensions
• Added test for exact example from issue: [x for x in x if x in [1]]

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

typeshed-stats (https://github.com/AlexWaygood/typeshed-stats)
- ERROR src/typeshed_stats/gather.py:999:28-1002:10: `Pattern[Unknown] | @5300 | None` is not assignable to upper bound `bytes | str` of type variable `AnyStr` [bad-specialization]
+ ERROR src/typeshed_stats/gather.py:999:28-1002:10: `Pattern[Unknown] | @7438 | None` is not assignable to upper bound `bytes | str` of type variable `AnyStr` [bad-specialization]

[meta-codesync]

@grievejia has imported this pull request. If you are a Meta employee, you can view this in D89812790.