workspace-indicator: Include n-workspaces in status label

The two extensions currently use a slightly different label
in menu mode:
The workspace indicator uses the plain workspace number ("2"),
while the window list includes the number of workspaces ("2 / 4").

The additional information seem useful, as well as the slightly
bigger click/touch target, so copy the window-list behavior.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/307>

extensions/workspace-indicator/workspaceIndicator.js

@@ -388,8 +388,9 @@ export class WorkspaceIndicator extends PanelMenu.Button {
     }
 
     _getStatusText() {
+        const {nWorkspaces} = global.workspace_manager;
         const current = this._currentWorkspace + 1;
-        return `${current}`;
+        return `${current} / ${nWorkspaces}`;
     }
 
     _updateMenuLabels() {