Need to display tabs correctly with tab-bar-format-tabs-groups under tab-bar-mode

```activities``` needs to respect ```tab-bar-mode```'s ```tab-bar-tab-group-face-function``` override in the same way it currently respects ```tab-bar-tab-face-function```. The lack of support means that the ```activities-tab``` face is never applied.

A PR coming shortly with the tested fix for this.

Configuration snippet for testing:
```elisp
(setq tab-bar-format
'(
tab-bar-format-tabs-groups ;; this shows groups
;; tab-bar-format-tabs ;; switch to this for plain tabs
tab-bar-separator
activate
tab-bar-format-align-right
tab-bar-format-global
))
```
With groups enabled, you'll have to assign a group to one tab to get the group to appear and for the broken formatting path to be invoked.

You can invoke this to ensure there's at least one group for testing ```(tab-bar-change-tab-group "*default*")```

or call ```M-x tab-group``` to assign a name interactively.

P.S. While reading through the ```tab-bar-mode``` code, it appears there is a bug that, if fixed, might render the PR correction unneeded. Perhaps in Emacs 30 if this is truly a bug and gets reported.
```elisp
(defun tab-bar-tab-group-face-default (tab)
(if (not (or (eq (car tab) 'current-tab)
(funcall tab-bar-tab-group-function tab)))
'tab-bar-tab-ungrouped
(tab-bar-tab-face-default tab)))
```
At the end of the function, ```tab-bar-tab-face-default``` is called and not ```tab-bar-tab-face-function``` so the override is never applied when groups are being formatted. Since the override would have been set to ```activities-tabs--tab-bar-tab-face-function``` the custom face is never applied. Also, to get around this, one can't mix both the tab format and the group format or the tabs get repeated.