Add XML files to default TextFile extensions in the editor

This allows opening `*.xml` files by double-clicking them in the
FileSystem dock.

editor/editor_settings.cpp

@@ -528,7 +528,7 @@ void EditorSettings::_load_defaults(Ref<ConfigFile> p_extra_config) {
 	// FileSystem
 	EDITOR_SETTING(Variant::INT, PROPERTY_HINT_RANGE, "docks/filesystem/thumbnail_size", 64, "32,128,16")
 	_initial_set("docks/filesystem/always_show_folders", true);
-	_initial_set("docks/filesystem/textfile_extensions", "txt,md,cfg,ini,log,json,yml,yaml,toml");
+	_initial_set("docks/filesystem/textfile_extensions", "txt,md,cfg,ini,log,json,yml,yaml,toml,xml");
 
 	// Property editor
 	EDITOR_SETTING(Variant::FLOAT, PROPERTY_HINT_RANGE, "docks/property_editor/auto_refresh_interval", 0.2, "0.01,1,0.001"); // Update 5 times per second by default.