i/5782: Implemented styles for .ck-button with a keystroke preview.

[oleq]

Suggested merge commit message (convention)

Feature: Implemented styles for .ck-button with a keystroke preview (see ckeditor/ckeditor5#5782).

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Additional information

Part of ckeditor/ckeditor5-ui#531.

[Reinmar]

The difference in styling between the label and the keystroke is IMO a bit too small, but I checked that the keystroke has the lowest possible contrast already, so there's not much room for improvement (by changing the color). Let's see how it will look in a real feature.

[oleq]

What you posted is not a real use-case because this kind of information will be used in drop–down menus only (does not make much sense in toolbars). And in that situation, there's some gap between the label and the keystroke, and some specific alignment, etc..