feat: Add leftTranslateByVector2 method to Matrix4

[luanpotter]

As a follow-up of #349, we might want to support leftTranslateByVector2, for the same reasons.

Note: there were no tests that I could find for the other broken down methods, so I added some assertions. Please let me know if they should be somewhere else or if we want more exhaustive tests.

Note: I am not implementing an equivalent for the scale method because the default implementation of falling back y and z to x felt less intuitive when using a Vector2. It would mean that scaling by a Vector2 v would be equivalent to scale by v.x, v.y, v.x, 1.0, which isn't quite as clear as the translation case. But I'd be happy to add the helper if desired.