[ty] Add opt-in `truthiness-test-of-none-union` rule (#28889)
## Summary
Add a new opt-in `truthiness-test-of-none-union ` rule which flags
truthiness checks on types like `int | None` or `str | None`. For
example, it will emit an error on the following (problematic) `if not
limit` check:
```py
def take(items: list[str], limit: int | None = None) -> list[str]:
if not limit: # error: [truthiness-test-of-none-union]
return items
return items[:limit]
```
Related to https://github.com/astral-sh/ruff/discussions/8757 and
inspired by Pyrefly's `implicit-bool` rule (but narrower in scope).
## Test strategy
New test suite
