Add missing arguments to Parameter and add type hints

[binste]

I noticed in #3143 that alt.param sets some attributes on alt.Parameter which do not exist on that class when its initialised. That's a problem for mypy as all attributes should exist from the beginning. This PR adds empty, param, and param_type and type hints for the whole class.

I tested with the Interval examples from the docs to make sure that indeed empty=Undefined leads to empty not being present in the final chart dict which is the old behaviour when empty was not present on the parameter instance at all if it was not defined.

[joelostblom]

Thank you @binste ! I am not very familiar with type but this looks harmless, so approving to avoid it going stale. Feel free to merge.

[binste]

Thanks @joelostblom :)