Defaults *.fs to F#

tests/no_duplicate_extensions.rs

@@ -14,6 +14,8 @@ fn no_duplicate_extensions() {
         // The "Ruby Haml" syntax also comes with a '.sass' extension. However, we make sure
         // that 'sass' is mapped to the 'Sass' syntax.
         "sass",
+        // The '.fs' extension appears in F# and GLSL. We default to F#.
+        "fs",
     ];
 
     let assets = HighlightingAssets::from_binary();