Editor: Add hook to disable the Font Library

[itzmekhokan]

Adds a developer hook to disable the Font Library, allowing sites to remove the feature from the admin interface. Useful for sites where fonts are managed by the theme and the ability to install fonts from the UI is not needed.

What the problem was:

• The Font Library (admin menu item + standalone admin screen, new in 7.0) is only gated by the edit_theme_options capability, which is shared with many unrelated features and cannot be repurposed to switch the feature off.
• There was no way for a developer to remove the Font Library from the interface for managers who hold admin rights but should not install fonts.

What the fix does:

• Introduces wp_is_font_library_enabled() and the wp_is_font_library_enabled filter (default true).
• When the filter returns false, the "Fonts" admin submenu item is not registered and the Font Library admin screen (font-library.php) returns a 403.
• Adds unit tests covering the default value, disabling via filter, and boolean casting of the filtered value.

Trac ticket: https://core.trac.wordpress.org/ticket/65550

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.8
Used for: Ticket analysis and tests cases. All changes were reviewed, validated against the codebase, and are taken responsibility for by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[github-actions]

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[t-hamano]

Thanks for the PR. As a pull request for this has already been submitted to the Gutenberg repository, let me close this.

WordPress/gutenberg#79027