Refactor the dispatcher

[bartfeenstra]

Replace all dispatching interfaces with plugins or a yet to be developed event API.

Once core components are defined as such, they should perhaps get an interface method that lets them register event handlers. We can invoke these methods after resolving the order of extensions.

Consider renaming events:

• loader -> ancestry load
• post loader -> ancestry process
• generate -> site generate

Document all events

Events can be types. Extensions can register handlers using these types, making everything type-safe

CssProvider and JsProvider can be called once when the Jinja2 environment starts?