[CMSP-695] Filter WP font_dir

[jazzsequence]

This PR adds our own handling for the WP Font Library.

This is designed to filter the font_dir (added in WP 6.5 / Gutenberg 17.6.0) to use an alternate path by default that, on Pantheon, is a writeable filesystem (meaning they can install fonts on production without having to commit them to the codebase). This behavior can be overridden by filtering the pantheon_modify_fonts_dir value.

Note that this is blocked by WordPress/gutenberg#58696. Behaviorally speaking, while the tests pass and the font directory is filtered appropriately, testing manually returns an invalid JSON response error (see Loom).

That doesn't mean that we can't merge this PR (we might decide we don't want to, or default the pantheon_modify_fonts_dir to false until WordPress/gutenberg#58696 is fixed). A version_compare determines whether to even load the namespace handling our WP Font Library modifications, so anyone using < WP 6.5 will remain unaffected even if we merge this and do a release anyway.

An approach to add our own filter to define the font directory was considered and decided against, since the user could just use the core font_dir filter. Instead, I made our filter fire earlier (priority 9) so a default font_dir filter will override our filter.

We're storing the value of wp_get_upload_dir into a global ($_pantheon_upload_dir) so we do not create an infinite loop caused by filtering inside of a filter (font_dir and upload_dir), see WordPress/gutenberg#58696 (comment).

TODO: Documentation for the new filter (and the WP filter) and descriptions of the behavior on Pantheon's environment. This will be done in a PR against the docs repo separately.

[pwtyler]

That doesn't mean that we can't merge this PR (we might decide we don't want to, or default the pantheon_modify_fonts_dir to false until WordPress/gutenberg#58696 is fixed)

With enough decisions at play here, will flip this to draft for now.

A version_compare determines whether to even load the namespace handling our WP Font Library modifications, so anyone using < WP 6.5 will remain unaffected even if we merge this and do a release anyway.

I considered for a minute if we need a means to support this for users using newest versions of g'berg on Core versions <6.4 but I think that can be documented with alternative means instead, if at all.

[pwtyler]

• Lots of repetition could be cleaned up
• fonts.php I think could be simplified but maybe I'm wrong

[jazzsequence]

Tests are passing on the add-more-helpers branch of wpunit-helpers. Once that's merged, this will be ready for re-review.