Add separate language mode for F# (#1333)

Since highlight.js has better support for F#, let's opt into that.
The CodeMirror mode is still 'mllike', however.

lib/constants.js

@@ -729,6 +729,13 @@ export const LANGUAGES = [
     mode: 'erlang',
     highlight: true,
   },
+  {
+    name: 'F#',
+    mime: 'text/x-fsharp',
+    mode: 'mllike',
+    short: 'fsharp',
+    highlight: true,
+  },
   {
     name: 'Fortran',
     mode: 'fortran',
@@ -870,7 +877,7 @@ export const LANGUAGES = [
     highlight: true,
   },
   {
-    name: 'OCaml/F#',
+    name: 'OCaml',
     mode: 'mllike',
     short: 'ocaml',
     highlight: true,