Add priest shadowform auto-paging, sharing the druid-stealth flag

Mirror the druid-stealth page switch for priests: swap to the auto page
(8) while in Shadowform, back to the default page when it drops. Driven
by UPDATE_SHAPESHIFT_FORM (form ID 28), which fully covers shadowform on
its own -- no prowl-style stealth substate to chase.

Fold the new state into the existing prowl machinery instead of
duplicating it: prowl/shadowform are mutually exclusive by class, so one
shared `formpaging` flag and one page constant serve both, and the
OnUpdate switch collapses to a single class-gated block.

env/translations_enUS.lua

@@ -788,6 +788,7 @@ pfUI_translation["enUS"] = {
   ["Switch Pages On Alt Key Press"] = nil,
   ["Switch Pages On Ctrl Key Press"] = nil,
   ["Switch Pages On Druid Stealth"] = nil,
+  ["Switch Pages On Priest Shadowform"] = nil,
   ["Switch Pages On Shift Key Press"] = nil,
   ["Systeminfo"] = nil,
   ["Target"] = nil,