[python-package] add type hints on plotting code

[jameslamb]

Contributes to #3756.

Adds type hints on most other places in python-package/plotting.py that were missing them.

Also proposes some other changes that I feel make it easier to follow the flow of data through that module (which I think is closely related to the goal of adding type hints).

• removes defaults in internal function plotting._to_graphviz()
  • to prevent "forgot to pass an argument through from the public interface" types of bugs
• switches to keyword arguments for any function calls passing more than two arguments
  • to prevent "passed these arguments in the wrong order" types of bugs

Notes for Reviewers

I might propose a change similar to #5704 (moving add() up to the module level so it isn't redefined every time _to_graphviz() is called) in a future PR, but decided not to do that here so that it was clear, from inspecting at the diff, what I'm proposing changing with type hints and keywords arguments.

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