Highlights of this patch - Displaying at-rules in the Outline panel when css files are selected. - Make sure filtering at rules in the outline panel work - Displaying and searching for at-rules in the Quick open panel - Stop triggering getClassSymbols and getFunctionSymbols when viewing css files. - parse the JS content on call getClassSymbols and getFunctionymbol with Js specific language. Differential Revision: https://phabricator.services.mozilla.com/D305023