Script Loader: Propagate fetchpriority from dependents to dependencies.
This introduces a "fetchpriority bumping" mechanism for both classic scripts (WP_Scripts) and script modules (WP_Script_Modules). When a script with a higher fetchpriority is enqueued, any of its dependencies will have their fetchpriority elevated to match that of the highest-priority dependent. This ensures that all assets in a critical dependency chain are loaded with the appropriate priority, preventing a high-priority script from being blocked by a low-priority dependency. This is similar to logic used in script loading strategies to ensure that a blocking dependent causes delayed (async/defer) dependencies to also become blocking. See #12009.
When a script's fetchpriority is escalated, its original, registered priority is added to the tag via a data-wp-fetchpriority attribute. This matches the addition of the data-wp-strategy parameter added when the resulting loading strategy does not match the original.
Developed in https://github.com/WordPress/wordpress-develop/pull/9770.
Follow-up to [60704].
Props westonruter, jonsurrell.
Fixes #61734.