[syntax-errors] `return` outside function (#17300)
Summary
--
This PR reimplements [return-outside-function
(F706)](https://docs.astral.sh/ruff/rules/return-outside-function/) as a
semantic syntax error.
These changes are very similar to those in
https://github.com/astral-sh/ruff/pull/17298.
Test Plan
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New linter tests, plus existing F706 tests.
