Support location_id in [teleport]

(cherry-picked from commit fed46ec45f)

changelog.md

@@ -23,6 +23,7 @@
  ### WML engine
    * Support formula= key in [variable] ConditionalWML
    * Support to_location in [move_unit], taking a location ID
+   * Support location_id in [teleport] for the target location
    * Support [filter_weapon] in leadership and resistance abilities,
      which activates the ability only when the affected unit is using
      a matching weapon.

data/lua/wml-tags.lua

@@ -866,6 +866,10 @@ wml_actions.teleport = function(cfg)
 		-- No error if no unit matches.
 		return
 	end
+	local x,y = cfg.x, cfg.y
+	if not x or not y then
+		x,y = wesnoth.special_locations[cfg.location_id]
+	end
 	wesnoth.teleport(unit, cfg.x, cfg.y, cfg.check_passability == false, cfg.clear_shroud ~= false, cfg.animate)
 end