Font Library: fix duplicate variants

[jffng]

What?

This PR adds checks to ensure that multiple file types of the same font variant are not added, so that only one variation per font style and weight is stored.

Why?

Fixes #54253

How?

For fonts where the font family does not yet exist, checks the list of new font faces and compares their font weight and style, and if there is already an existing font face, skips adding the incoming one.

For font faces where the font family exists, adds a new function get_intersecting_font_faces which returns an array of font faces to be deleted and replaced by the incoming font face.

Testing Instructions

1. Simultaneously (batch) upload two or more font faces of the same family, style, and weight, but different file types.
2. Ensure only the first variant is added / used.
3. Upload a font face to an existing font family with the same style and weight, but different file type.
4. Ensure the uploaded version is used / replaces the existing one, and an additional variant is not created.

Testing Instructions for Keyboard

Screenshots or screencast

[jffng]

Should I add a test for this?

[matiasbenedetto]

I think we are not testing private class methods, and I also don't know if there is an easy way to do that and if we should do it. I would prefer to add a test case(s) for the public method (install) that covers the situations we are trying to fix with the changes in this PR.

[github-actions]

Flaky tests detected in 83bff3d.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6186790338
📝 Reported issues:

• [Flaky Test] should partially select with shift + click (@webkit) #53524 in /test/e2e/specs/editor/various/multi-block-selection.spec.js

[jffng]

The rebase on this PR got ugly after the frontend was merged, so I opened a new PR at #54490.

Closing this one.