Whitelist hideFromUser key in grammars (#3989)

hideFromuser is an undocumented key from TextMate to hide some
grammars from the user.

tools/grammars/compiler/data.go

@@ -26,4 +26,5 @@ var KnownFields = map[string]bool{
 	"foldingStartMarker": true,
 	"foldingEndMarker":   true,
 	"limitLineLength":    true,
+	"hideFromUser":       true,
 }