Follow-up for #3198297: automate plugin icon extraction, button label, etc.

Problem/Motivation

The svg for the icons in the Admin Toolbar UI added in [] was manually copied. In the future it would be better if there was a way to automate this, both for this project and any contrib that used icons that are embedded in the plugin code / repo.

Proposed resolution

@reinmar suggested a possible way forward, using an instantiated editor with the plugin added, calling:
editor.ui.componentFactory.create('toolbarItemName')
This returns an object with a number of useful methods, specifically a getter for icon
which returns a string of the svg.

The ckeditor 4 module in core uses a "hidden" fully loaded instance of ckeditor to get the icons for the toolbar,
we could do something similar in a build step- load the editor with the given plugin(s) and write the output svg to a file.

This same method may be useful for getting translated labels as well, since there is also a getter for the tooltip string.

Remaining tasks

• Determine the best way to automate future icon changes / additions / replacements.
• Determine how to use the icon, whether that is saved to a file, loaded dynamically in the toolbar ui etc.
• Decide whether to also get the tooltip label the same way so that tooltips are consistent and also take advantage of CKEditor 5's translation system