feat: Easily add custom types

[Reathe]

This is more of a question, because I'm not sure if this is possible, sorry if I missed something.

Let's say I wrote a wezterm plugin for my personal use, and I want to have type hints for it.
Obviously it doesn't make sense to add the types for it to this repo since basically no one will benefit from it.
So I thought it would be nice to be able to add types for a plugin locally.

[DrKJeff16]

Any type annotations are just as valid to add here, if you want!

[DrKJeff16]

@Reathe I took the liberty to move this to the Discussions tab.