Button: Standardise text labels for icon buttons

[getdave]

For buttons which are displayed as icons only, it's important that they have a text based alternative which can be toggled on/off via editor preferences under Appearance -> Show button text labels.

Currently it appears the CSS required to do this needs to be reimplemented for each location. It would be helpful if the component-side of this was standardised in the base component.

#47930 is an example of ensure a given UI supports text only labels.

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@tellthemachines In a followup it feels like we should make this standardised CSS somehow.

@ciampo This CSS allows us to use text alternatives for icon buttons. Is it possible to bake this into the component in @wordpress/components?

Originally posted by @getdave in #47930 (comment)

[tellthemachines]

We haven't standardised this yet because the rollout of text labels has been progressive, and there are parts of the editor where design work is still needed to rethink component layout to support text labels. Once we have them working across the whole interface, then we might be able to refactor the CSS to apply globally, but currently that would break too much of the UI. See #46299 for more discussion on this!