Font Library: Add post types and low level APIs

[youknowriad]

This PR backports the font library post types and low level APIs to Core. This is the first step to include the font library entirely into Core. Once this merged, we'll open a PR with the necessary REST API controllers.

Trac ticket: https://core.trac.wordpress.org/ticket/59166

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[getdave]

PHPCS errors are mainly tabs vs spaces. Should be simple to clean up.

[youknowriad]

Actually, it looks like I opened a PR on the wrong repo (core instead of my own fork) I'm going to close this one and open a new PR soon.

[youknowriad]

Closing in favor of #6027