Use 'theme_files' cache group for block pattern caches. Previously, block pattern cache data was not stored in a cache group and used the default group. This new cache group, is setup as a global cache group, meaning that sites using multisite, will have a single cache for block pattern data per theme. This change also no longer invalidate block pattern caches in multisite instances, meaning block pattern caches can be shared between sites on a network, meaning less repeated data in the object cache.
Props spacedmonkey, flixos90, joemcgill.
Fixes#60120.
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