Add class keyword to Class Types (#56)

The class keyword was forgotten. This commit adds it to the possible
class types. This class type is mostly used to extend ExternalObject.

syntaxes/metamodelica.tmGrammar.yaml

@@ -88,7 +88,7 @@ patterns:
     name: support.type
 
   # Class Type
-  - begin: \b(record|type|package|function|uniontype)\s+(\w+)\s*("([^"]|\\")*(?<!\\)")?
+  - begin: \b(class|record|type|package|function|uniontype)\s+(\w+)\s*("([^"]|\\")*(?<!\\)")?
     beginCaptures:
       1:
         name: keyword