feat: #3015812 Introduce new Theme extension object and properly deprecate...

feat: #3015812 Introduce new Theme extension object and properly deprecate REGIONS_VISIBLE and REGIONS_ALL

By: alexpott By: andypost By: phenaproxima By: markhalliwell By: vacho By: voleger By: berdir By: hardik_patel_12 By: nikitagupta By: jofitz By: smustgrave By: quietone By: viappidu By: ameymudras By: claudiu.cristea By: ravi.shankar By: suresh prabhu parkala By: akram khan By: nicxvan By: xjm By: godotislate

Closes #3015812