dev: use WP_Abilities_Registry::$instance instead of adding a new global

[justlevine]

What

This PR changes WP_Abilities_Registry to store it's instance on ::$instance, instead of on a new global $wp_abilities variable.

(Note: I'm just code-reviewing and leaving feedback. If you think this warrants a wider discussion I can open a matching issue, otherwise I think a discussion here following by [accept | reject] is enough)

Why

Global variables are hard-to-type-and-test antipatterns. Just because legacy WP code used them before there were better patterns doesn't mean we need to introduce new ones.

Using a private instance gives core contributors and end-devs more insight into usage, and as developers working on the Abilities API, we get a much smaller surface for breaking changes to worry about.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (trunk@507e9ec). Learn more about missing BASE report.

Additional details and impacted files

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##             trunk      #19   +/-   ##
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  Coverage         ?   90.64%           
  Complexity       ?       99           
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  Files            ?        7           
  Lines            ?      556           
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  Hits             ?      504           
  Misses           ?       52           
  Partials         ?        0           

Flag	Coverage Δ
unit	90.64% <100.00%> (?)

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[gziolo]

One of the unit tests is failing with these changes:

Unfortuntalty, the existing CI check doesn't propagate test failures and all jobs still pass.

[gziolo]

I don't have a strong preference here. WordPress uses globals for many fundamental singletones like $post, $wp_scripts, $wp_rest_server. However, we need to make it fully testable and be able to reset the instance between test runs.

[justlevine]

Unfortuntalty, the existing CI check doesn't propagate test failures and all jobs still pass.

Also unfortunately, our PHPUnit CI is passing when the tests itself fail - I'll take a look at that too! 😅

[gziolo]

Using reflection makes the test pass 👍