Cleanup: Rename ARegion variables from ar to region

The old convention was easy to confuse with ScrArea.
Part of https://developer.blender.org/T74432.

This is mostly a batch rename with some manual fixing. Only single word
variable names are changed, no prefixed/suffixed names.

Brecht van Lommel and Campbell Barton both gave me a green light for
this convention change.

Also ran clan clang format on affected files.

source/blender/python/intern/bpy_rna_callback.c

@@ -58,7 +58,7 @@ static const EnumPropertyItem region_draw_mode_items[] = {
     {0, NULL, 0, NULL, NULL},
 };
 
-static void cb_region_draw(const bContext *C, ARegion *UNUSED(ar), void *customdata)
+static void cb_region_draw(const bContext *C, ARegion *UNUSED(region), void *customdata)
 {
   PyObject *cb_func, *cb_args, *result;
   PyGILState_STATE gilstate;