fix(no-useless-undefined): Support functions with allowed undefined return types and mixed returns

[nsotiroglou96]

This update refines the no-useless-undefined rule to better handle cases where returning undefined is valid, specifically:

1. The rule now inspects the enclosing function’s return type annotation and skips reporting when that type includes undefined (e.g. Promise<number | undefined>).
2. It also introduces an allowMixedReturns option so that functions with mixed return values (some branches returning a value, others undefined) aren’t flagged unnecessarily.

[sindresorhus]

Please make sure npm test passes locally.

You need to add tests.

[sindresorhus]

Bump :)