Add Icon APIs: registry and /wp/v2/icons endpoint

[mcsf]

Companion PR to Gutenberg's WordPress/gutenberg#72215 and WordPress/gutenberg#74943

Builds on #10795

Trac ticket:

[github-actions]

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[fabiankaegy]

I do not think we should merge in this API without a public method for registering icons.

The icon library we are shipping with Core is the gutenberg icons. Those icons were never meant to be frontend design icons. They are editor interface icons.

From my POV there is very little utility to the icon block in core without the ability to register custom icons.

[mcsf]

I understand where you're coming from, but I'd argue that the Icon block is already useful in its initial form, having access to 321 core icons.

Though @wordpress/icons was developed to support the needs of the editor, IMO it's fair to say that the icon set can accommodate more uses. Folks like @jasmussen, @jameskoster and others have been iterating on those icons and are involved in the making of the Icon block, which I take as a sign that, though imperfect, the icon set has evolved beyond its original purpose.

Keeping the Icons registry closed was a way to conserve some more room for adjustments, as the icon block itself gets adopted. It should be understood as an intermediate step, so that we can confidently expand the scope of the registry in 7.1 (third-party registration, but possibly also categories, etc.).