Rename get_time_stamp to ms_since_init

This better reflects what it actually does - it is not and has never been a timestamp.

data/lua/core/_initial.lua

@@ -73,6 +73,7 @@ end
 
 unpack = wesnoth.deprecate_api('unpack', 'table.unpack', 3, '1.17', table.unpack)
 math.pow = wesnoth.deprecate_api('math.pow', '^', 3, '1.17', function(a,b) return a ^ b end)
+wesnoth.get_time_stamp = wesnoth.deprecate_api('wesnoth.get_time_stamp', 'wesnoth.ms_since_init', 1, nil, wesnoth.ms_since_init)
 if wesnoth.kernel_type() == "Game Lua Kernel" then
 	-- wesnoth.wml_actions.music doesn't exist yet at this point, so create a helper function instead.
 	wesnoth.set_music = wesnoth.deprecate_api('wesnoth.set_music', 'wesnoth.music_list', 1, nil, function(cfg)

src/scripting/lua_kernel_base.cpp

@@ -349,7 +349,7 @@ static int intf_get_image_size(lua_State *L) {
 * Returns the time stamp, exactly as [set_variable] time=stamp does.
 * - Ret 1: integer
 */
-static int intf_get_time_stamp(lua_State *L) {
+static int intf_ms_since_init(lua_State *L) {
 	lua_pushinteger(L, SDL_GetTicks());
 	return 1;
 }
@@ -456,7 +456,7 @@ lua_kernel_base::lua_kernel_base()
 		{ "name_generator",           &intf_name_generator           },
 		{ "log",                      &intf_log                      },
 		{ "get_image_size",           &intf_get_image_size           },
-		{ "get_time_stamp",           &intf_get_time_stamp           },
+		{ "ms_since_init",            &intf_ms_since_init           },
 		{ "get_language",             &intf_get_language             },
 		{ nullptr, nullptr }
 	};