Document Marker event subscription methods
Document TextBuffer event subscription methods
Make Marker event subscription methods public

Nothing else has the extended/essential distinction currently and this
isn’t the purpose of this documentation pass.
Deprecate all usages of Marker::on
Add Marker::once
Deprecate TextBuffer::on/once
Upgrade event-kit to provide deprecated .off method on disposables

This allows us to use the new event subscription methods in our
deprecated ::on shim. They still won’t support namespaces and ::off,
so we’ll have to see what happens there.