General: Introduce WP_DEVELOPMENT_MODE constant to signify context-specific development mode.
In recent releases, WordPress core added several instances of cache usage around specific files. While those caches are safe to use in a production context, in development certain nuances apply for whether or not those caches make sense to use. Initially, WP_DEBUG was used as a temporary workaround, but it was clear that a more granular method to signify a specific development mode was required: For example, caches around theme.json should be disabled when working on a theme as otherwise it would disrupt the theme developer's workflow, but when working on a plugin or WordPress core, this consideration does not apply.
This changeset introduces a WP_DEVELOPMENT_MODE constant which, for now, can be set to either "core", "plugin", "theme", or an empty string, the latter of which means no development mode, which is also the default. A new function wp_get_development_mode() is the recommended way to retrieve that configuration value.
With the new function available, this changeset replaces all existing instances of the aforementioned WP_DEBUG workaround to use wp_get_development_mode() with a more specific check.
Props azaozz, sergeybiryukov, peterwilsoncc, spacedmonkey.
Fixes #57487.