Unmatched parenthesis in Do block confuses syntax highlighting

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!