Offer a z-index appending function for plugin use

[meowsus]

Is your feature request related to a problem? Please describe.
Plugins that add UIs which are intended to display over other UIs (such as Search Autocomplete, modals, etc) have a hard time hooking into the base z-index system.

Describe the solution you'd like
Without spending a ton of time thinking up solutions, perhaps providing a function, much like this one, would be helpful. Ideally plugins would be able to append their z-index name and have it affect the whole app.

This way a plugin could append its name to the beginning, middle, or end of the sass list and base/all other plugins would be affected accordingly.

Describe alternatives you've considered
The search-autocomplete component inside the workarea-search-autocomplete plugin contains code that cracks into the $page-container z-index list to append itself to the end of the list. This works because it's reactionary, but it would be nice to have a solution baked into base to avoid this boilerplate code in the future.

[meowsus]

This isn't a real issue facing any known build, yet.