Support [variables] in [modify_side]

This allows merging in changes to side variables and may alleviate the lack of [set_variables] support for side variables

changelog.md

@@ -47,6 +47,7 @@
    * Support upkeep in StandardUnitFilter
    * [effect]apply_to=variation now supports heal_full
    * Support [set/clear_variable] inside [modify_unit/side]
+   * Support [variables] in [modify_side], as in [modify_unit]
  ### Packaging
    * The Wesnoth client now looks for the data/dist file when logging into the multiplayer server.
      This file should contain one of the following values based on where the package is for:

data/lua/wml/modify_side.lua

@@ -103,6 +103,8 @@ function wesnoth.wml_actions.modify_side(cfg)
 				wesnoth.wml_actions.set_variable(v[2], side.variables)
 			elseif tag == "clear_variable" then
 				wesnoth.wml_actions.clear_variable(cfg, side.variables)
+			elseif tag == "variables" then
+				side.variables.__cfg = wml.merge(side.variables.__cfg, content, cfg.var_merge_mode or "replace")
 			end
 		end