workspaceAnimation: Make WorkspaceGroup public

workspaceAnimation: Make WorkspaceGroup public

The WorkspaceGroup class in defined as CONST, which means that,
strictly speaking, is inaccessible from outside the file
workspaceAnimation.js. But Desktop Icons NG needs access to it.

Although the current Javascript engine "tolerates" this access,
a warning message is shown in the log advertising that it's
incorrect, and that although it is still allowed, the code
should be fixed.

This patch changes the definition from CONST to VAR to allow
accessing it from extensions.

jk

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2068>