[pylint] Implement singledispatchmethod-function (PLE5120)

[augustelalande]

Summary

Implement singledispatchmethod-function from pylint, part of #970.

This is essentially a copy paste of #8934 for @singledispatchmethod decorator.

Test Plan

Text fixture added.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[charliermarsh]

Thanks!