environment: Add async ease variants

environment: Add async ease variants

Ease functions can be used nested, and often need to complete before
performing an action.

This is currently handled by providing onComplete/onStopped callbacks,
which can get tedious when nesting.

To make these cases more convenient, add async variants that return
a promise that is resolved when the transition completes, so that
callers can await the result instead.

In case the transition was interrupted, the promise is rejected with a
CANCELLED error, so that it stops a promises chain and can easily be
filtered out in catch handlers.

Base on an original patch by Marco Trevisan.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3948>