Unmatched parenthesis in Do block confuses syntax highlighting

[frangio]

Vader syntax highlighting is broken for scenarios like:

Do:
  i(-
Expect:
  (-)

The opening parenthesis in the Do block causes Vader syntax to be ignored until a matching closing parenthesis is found.

I tried briefly to fix it but I'm inexperienced in writing syntax descriptions for Vim. Would love to see how it can be done!