Add support for Speculative Loading WP Core API, loading the plugin's own API implementation conditionally

[felixarntz]

Summary

Following the WordPress Core merge of speculative loading last week, this PR ensures the Speculative Loading plugin uses the WP Core API when available, and only loads its own API implementation if not.

Relevant technical choices

• Split code paths between one file for the plugin API and another one for the WP Core API integration.
• Load these files conditionally, one or the other (for the plugin API, this also includes the PLSR_URL_Pattern_Prefixer class), and only add the relevant hooks.
• For PHPUnit, all files are still unconditionally loaded to keep all code tested. The only thing that is conditional in tests is which hooks are added, which is the only real differentiator for which implementation is used.

The fact that WP Core trunk tests pass with this is a good indicator this works as expected. Also check in the PHPUnit test run using trunk on GitHub Actions that for Speculative Loading (50 tests) it makes 1 assertion more than for the other WordPress versions (104 instead of 103) because of the one additional hook checked.

[github-actions]

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Co-authored-by: felixarntz <[email protected]>
Co-authored-by: joemcgill <[email protected]>

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[felixarntz]

Note for reviewers: This function was moved without any changes to the plugin-api.php file.

[felixarntz]

Note for reviewers: These tests for the plsr_print_speculation_rules() function were moved to test-speculation-rules-plugin-api.php without any changes.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 66.79%. Comparing base (5693cc4) to head (08ae13e).

Files with missing lines	Patch %	Lines
plugins/speculation-rules/hooks.php	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1883      +/-   ##
==========================================
+ Coverage   66.70%   66.79%   +0.08%     
==========================================
  Files          88       89       +1     
  Lines        7029     7044      +15     
==========================================
+ Hits         4689     4705      +16     
+ Misses       2340     2339       -1     

Flag	Coverage Δ
multisite	66.79% <75.00%> (+0.08%)	⬆️
single	37.39% <60.71%> (+0.19%)	⬆️

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[joemcgill]

These changes look good to me.