Adding type hints to Param's source code

[maximlt]

These days more and more people ask library maintainers to add type hints to their code base, so I'm doing it before someone else gets the chance to do it! 🙃

Besides that, I wouldn't be surprised if the process of adding type hints to the source code would uncover a few bugs, which wouldn't be a bad thing. It would also improve the user experience when writing Param's code in an editor supporting type hints like VS Code.

Also, if the holoviz group decides to gradually add type hints to their packages, this is probably the place where it should start.

Of course, this work needs to wait until Python 2 support is dropped, which I hope will happen soon.

I hope in the future to be able to add more information to this issue about the best way to add type hints to a code base and raise the major difficulties (or blockers if any) that could be encountered.