Ignore cached wp_theme_has_theme_json when WP_DEBUG is enabled

[mmtr]

Follows up #45543

What?

This PR updates the wp_theme_has_theme_json so it ignores the cache when WP_DEBUG is enabled.

Why?

• To ensure that the theme developers workflow is not affected by the cache (otherwise they might get stale styles when adding/removing a theme.json file).
• For consistency with how the cache is (or will be) ignored by other areas (see Global Styles: Update gutenberg_get_global_stylesheet to use WP_Object_Cache #45679 and Add WP_Object_Cache to the gutenberg_get_global_settings method #45372).

Testing Instructions

TBD

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[felixarntz]

LGTM in terms of just fixing the problem here, although we may want to consider a more holistic situation.

How about introducing a function like wp_theme_debug_enabled() which returns this as a boolean, which we can use in any places where we use caching around theme-powered code? At a minimum the function could be the WP_DEBUG check we've been starting to use, but potentially it should go further with e.g. additionally having a filter to control it so that it can be dynamically set or modified at runtime.