Make it so languages can specify a path to load a mode from. Add correct SCSS mode definition.

For #3726. We were incorrectly using SASS mode for SCSS files. SCSS is defined as a mime-type of the "css-base" mode, which is also the base mode for the "css" mode. LanguageManager currently assumes that a mode's JS file has the same name as the mode, which isn't true for "css-base". So I also added the ability to specify an optional extra argument to the mode specifier for the path (within the mode folder) to the mode's file.