feat: Add compat/client types

[rschristian]

Closes #3845 (but for real this time)

Semi-related: is it intentional that Preact can take a ComponentChild as a render arg whilst compat can only take VNode<any>? Doesn't seem like there's an immediately obvious issue w/ compat rendering a string, etc., as it's passed straight through to the core render(), more or less.

[coveralls]

coverage: 99.605%. remained the same
when pulling bb064a3 on types/compat-client
into f0c3e1d on main.

[JoviDeCroock]

You are right yes, that should be possible