workspace: Use Util.lerp() instead of actor box for interpolating

workspace: Use Util.lerp() instead of actor box for interpolating

Instead of interpolating our workspace and layout boxes for each child
using clutter_actor_box_interpolate(), use our Util.lerp() function and
stay in JS land instead.

This is quite a large performance improvement since it avoids
heap-allocating a new ClutterActorBox for every child. With this, we're
finally at a duration of 1.0 ms to allocate the Workspace with 20
windows.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1743>