Typing support

[fennerm]

I'm interesting in working on this. Would you accept a PR for it?

Is your feature request related to a problem? Please describe.
Type checking for code which uses argdantic isn't possible.

Describe the solution you'd like

• Fix mypy errors in the project
• Add mypy to pre-commit config
• Add py.typed file

Describe alternatives you've considered
Open to using other type checkers as well if you'd prefer.

[edornd]

Tried my best, will certainly add mypy to the pre-commit configuration, but for the moment I fixed major typing problems (where it seemed necessary)!