[processing] add output extent to scripts (refs #11781)

Author: alexbruy

python/plugins/processing/script/ScriptAlgorithm.py

@@ -293,6 +293,8 @@ def processOutputParameterToken(self, token):
             out = OutputNumber()
         elif token.lower().strip().startswith('string'):
             out = OutputString()
+        elif token.lower().strip().startswith('extent'):
+            out = OutputExtent()
 
         return out