docs: Add JSDoc for all events

The official JSDoc events is confusing and poorly documented, so this is really just “tagging” where event are listened for, and where they are emitted.

…That said, I think it’s a good start towards documenting the public API of Eleventy’s internals. JSDocs like this—even when they exist only in the source—will be particularly helpful for plugin authors who want to explore what they are able to do, and how they might do it.