Fixes for autocompletion within comments, roxygen

This PR brings some fixes:

1. We ensure that completions after a `$` or `@` never gets completions from outside contexts,
2. We never provide completions within comments (we may want to tweak this in the future to only disable automatically popped up completions, but allow triggered completions),
3. We only provide Roxygen completions within an roxygen comment (may want to modify similar to above later)