[red-knot] Type narrowing for `isinstance` checks (#13894)
## Summary
Add type narrowing for `isinstance(object, classinfo)` [1] checks:
```py
x = 1 if flag else "a"
if isinstance(x, int):
reveal_type(x) # revealed: Literal[1]
```
closes #13893
[1] https://docs.python.org/3/library/functions.html#isinstance
## Test Plan
New Markdown-based tests in `narrow/isinstance.md`.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
