WP_Theme Revamp and Improving Report 63086

[SirLouen]

I was going crazy with so many repetitions of the same code in the WP_Theme class. Extremely difficult to understand the constructor and to track down the problems. But still, I think that comments are not very self-explanatory.

This patch partially refactors the constructor and sorts this issue (that is mostly caused because of the current convoluted state of such constructor). And still, I'm not 100% sure that this is the best solution.

Apart from this, here are the testing instructions for this patch:

1. I have created an adhoc plugin that checks the result of wp_is_block_theme() function in each step
2. Install this plugin as a mu-plugin
3. Install this Child theme and activate it
4. Make sure you have TwentyTwentyFive also installed
5. The theme should be loading without issues.
6. Activate a non-block theme ⇾ Go to customizer, check there are no notices there
7. Add the short code [block_theme_detection] to a post and check if its passing in all the actions hooks

Trac ticket: https://core.trac.wordpress.org/ticket/63128

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Props sirlouen.

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