What problem does this address?

The library's name originally was coined as singular form, i.e. "Font Library". Along the way, it changed to the plural form of "Fonts".

The plural form aligns well to font foundries like Google Fonts, Adobe Fonts, Bunny Fonts, etc.

The singular form aligns well to other WordPress-specific libraries and directories, such as:

• Block Library
• Pattern Library
• Media Library
• Photo Directory

Props to @ironprogrammer for raising the singular form relationship to the above list.

What is your proposed solution?

Rename to use the singular form. Instead of "Fonts Library", rename to "Font Library".

Changes:

• The merged PHP backend files:
  • lib/experimental/fonts/fonts-library becomes lib/experimental/fonts/font-library.
  • WP_Fonts_Library becomes WP_Font_Library and its filename changes too.
  • WP_REST_Fonts_Library_Controller becomes WP_REST_Font_Library_Controller plus its filename.
  • and the associated tests.
• All open issues.
• Rename in frontend work Font Library: Frontend #53650.

Part of #52698.