Modernization PR

[mkhorton]

This will address some of the items in #265. In brief, several choices were made early on in Crystal Toolkit development due to the limitations of Dash v1. Some of these choices can now be re-thought due to features now available in Dash 2.

In addition, several aspects of Crystal Toolkit are either poorly documented or incomplete, and I would like to make sure these are polished.

So far, this PR includes:

• A new CrystalToolkitPlugin to replace the old register_crystal_toolkit() method.
• Comprehensive addition of all Bulma "Element" and "Form" classes with explicit kwargs, type hints and docstrings

Notifying the MP infra team (@tschaume, @yang-ruoxi, @munrojm, @janosh, @tsmathis). I'm making every effort to make sure no backwards incompatible changes are introduced. It's possible some backwards incompatible changes will be required, but if so I'll try and make these minimal and make sure that a migration path is easy and documented.

[mkhorton]

This PR will also:

• Switch to using the Jupyter Dash embedding (available from Dash 2.11+) for displaying components inline in Jupyter notebooks. I anticipate this to be a lot easier to maintain than our current extension, as well as supporting more Jupyter environments (notebook/VS Code), but with some slight performance penalty.

[mkhorton]

I intend to merge this PR once tests pass (which I believe are just Python 3.9 related) since the PR is otherwise complete, and should not contain any breaking changes. I've updated the documentation. I expect an additional "Modernization" PR to follow at a later date.