Introduce a new class, EventEmitter, which implements signal
handling for pure JavaScript classes. EventEmitter still
utilizes GJS' addSignalMethods internally.
EventEmitter allows static typechecking to understand the
structure of event-emitting JS classes and makes creating
child classes simpler.
The name 'EventEmitter' mirrors a common name for this pattern
in Node and in JS libraries.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2043>
