Summary

This PR can be summarized in the following changelog entry:

• Makes sitemap rewrite rules more reliable by applying them dynamically.

Relevant technical choices:

• A class Yoast_Dynamic_Rewrites is introduced as a layer to handle dynamic rewrites.
• Dynamic rewrites can be added with its add_rule() method, that has the same signature as core's WP_Rewrite::add_rule() (and its alias add_rewrite_rule()).
• The class filters the rewrite_rules option, adding all dynamic rewrite rules in there.
• The class still calls core's WP_Rewrite::add_rule(). This needs to happen so that rewrite rules are also in place when they are not fetched from the option, but being regenerated "live".
• In order to keep the dynamic rewrite rules out of the actual option (they are added via a filter and thus shouldn't redundantly pollute the option value), the class also filters sanitizing the rewrite_rules option, stripping those dynamic rules out of it.
• The class is instantiated on setup_theme since this is the first action where WP_Rewrite is available.

Please read the issue #10801 for more background information.

Test instructions

This PR can be tested by following these steps:

• Deactivate the plugin.
• Go to the Settings > Permalinks screen and just hit Save (you don't need to make any changes). This will flush rewrite rules (just to make sure Yoast SEO rewrite rules are not included).
• In the _wpseo_activate() function in wp-seo-main.php, temporarily comment out the if-else construct that flushes rewrite rules (starts with is_multisite() && ms_is_switched()).
• Activate the plugin again.
• Access sitemap_index.xml and ensure it appears, not giving a 404.
• From there, also access the other sitemaps, and ensure they appear.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #10801