This release is about extending Hyperdiv.

• Adds plugins, the ability to create new components with custom Javascript and CSS. Note that this new plugin interface breaks the previous, undocumented plugin interface.
• Enables extending the index page with top-level Javascript and CSS assets.
• Documents how to extend existing Hyperdiv components with new CSS props.
• Documents how to create new simple components that target HTML tags that Hyperdiv doesn't support out of the box.

This release accidentally breaks support for python 3.9, and requires 3.10+, due to using a glob feature in plugins that is only present in 3.10. The docs app won't load with 3.9. I may either re-introduce support for 3.9+ in the future, or make future versions uninstallable with 3.9.