Add type annotations

## Request

Add type annotations to hvPlot where ever it makes sense or is possible to do.

## Motivation

Type annotations make it much easier for (some/ a lot of) users to understand and work with functions and classes in Python. In many ways hvPlot is the simplest HoloViz project to add this too as the api is flat.

## Additional info

I would like be willing to add some if I new that the project would be willing to move in that direction. Is it @philippjfr and others?

## Example

There is no reason why for example the `explorer` should not be type annotated. It would help VS Code users a lot.