[flake8-pyi] Make example error out-of-the-box (PYI042)

[MeGaGiGaGon]

Summary

Part of #18972

This PR makes snake-case-type-alias (PYI042)'s example error out-of-the-box

Old example

type_alias_name: TypeAlias = int

New example

from typing import TypeAlias

type_alias_name: TypeAlias = int

Imports were also added to the "use instead" section.

Test Plan

N/A, no functionality/tests affected

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.