Fonts: Normalize CSS font face font-family name on font upload.

[sirreal]

I attempted to use a CSS font family on upload, but that results in broken display: WordPress/gutenberg#76253

Trac ticket: https://core.trac.wordpress.org/ticket/63568

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[github-actions]

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[sirreal]

This description is wrong, or at least is isn't what the Gutenberg client is sending.

In WordPress/gutenberg#76253 I sent over a real CSS value, and that resulted in the readable font name including CSS escapes.

Either way, it seems best to do this on the server.

The actual data that should be sent here is a plain string, not CSS. For example:

• ✅ My Font Name is an appropriate string to send
• 🚫 "My Font\20 Name" is a CSS string - do not send this!

[sirreal]

Alternative to #9951

[sirreal]

Same #11193 (comment)