Add ability to change individual icons in an icon library

[claviska]

This has been on my wishlist for awhile, and apparently others have interest too.

While the aforementioned PR works, we probably want to expose a more obvious API for changing individual icons in a library. Something like this would probably be easier to document and it would give us way to tweak the underpinnings if needed later on.

The API might look something like this:

import { setLibraryIcon } from '/dist/utilities/icon-library.js';

// function name tbd
setLibraryIcon('system', {
  resolver: name => `/path/to/custom/icons/${name}.svg`
});

Leaving this here as a TODO because we definitely want to make this easier.

[ak-beam]

• As well as documenting what icons are used in the system icon set in order to send a list to design for them to override