Webfonts API: Filter out enqueued webfonts not used in frontend [DO NOT MERGE]

[jeyip]

Dependent on #39327

What?

Why?

How?

Testing Instructions

Screenshots or screencast

[Mamaduka]

I'm doing some old PR cleanup. Since there has been no activity in the last three years, I'm going to close this as "stale."

[github-actions]

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