Add on demand block style loading in classic themes

[sjapaget]

Use output buffering for template enhancement to reduce unecessarily loaded CSS in classic themes. Makes use of wp_should_output_buffer_template_for_enhancement filter to delay the output of CSS until after the content is known and can be output by capturing the output of print_late_styles().

Trac ticket: https://core.trac.wordpress.org/ticket/64099

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[github-actions]

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Props sjapaget, westonruter.

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[github-actions]

Hi @sjapaget! 👋

Thank you for your contribution to WordPress! 💖

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[westonruter]

Thank you for working on this!

[westonruter]

We also need to add unit tests.

[westonruter]

@sjapaget Do you anticipate being able to continue working on this before next week? The beta1 release is on Tuesday, so this needs to be completed quickly. If you don't have availability, I'll follow up with additional commits to get it to the finish line.

[sjapaget]

@sjapaget Do you anticipate being able to continue working on this before next week? The beta1 release is on Tuesday, so this needs to be completed quickly. If you don't have availability, I'll follow up with additional commits to get it to the finish line.

@westonruter I've done a bit more to it this afternoon (just pushed up) and will have time tomorrow too, but I'd very much appreciate some pointers on how to approach testing this properly if possible?