Boost: Gracefully handle overflow of max concat files

[haqadn]

Fixes HOG-17
Fixes #40365

Proposed changes:

• Introduce a new filter to control the maximum number of files allowed to be concatenated, jetpack_boost_minify_concat_max_files
• If a CSS/JS group is bigger than the allowed number of files, chunk them into groups of the allowed size, default 150.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Activate minify JS/CSS and create a page with a lot of CSS or JS files enqueued. This part is manual and I don't have a good suggestion to achieve. I guess install a few plugins that add front-end JS and CSS.
• Use the following filter to customize the number of CSS/JS files:

add_action( 'jetpack_boost_minify_concat_max_files', function( $_max_files ){ return 3; } );

• Ensure that the concatenated files (_jb_static, and boost-cache/static) are grouped by less than or equal to the given number of files given above '3'.

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Code Coverage Summary

Coverage changed in 3 files.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/boost/app/lib/minify/class-concatenate-css.php	0/99 (0.00%)	0.00%	2 ❤️‍🩹
projects/plugins/boost/app/lib/minify/class-concatenate-js.php	0/145 (0.00%)	0.00%	2 ❤️‍🩹
projects/plugins/boost/app/lib/minify/functions-helpers.php	0/140 (0.00%)	0.00%	1 ❤️‍🩹

Full summary · PHP report · JS report

[dilirity]

I don't think this is working (for CSS at least).

I've added the filter with 3 max files, but the page is broken.

Here's the source of concatenated files:

I added elementor and a couple of blocks to make it output a lot of stylesheets. Just to make sure my tests are okay, I switched to trunk and everything works.

[dilirity]

Awesome! CSS now works as well. Let's merge this!