- Match comment type include and annotation region.
- Allow FlowType syntax inside of these regions.
- Highlight single colon to represent special meaning (kept verbosely in
  type annotation).

This commit adds support for flow annotations to statements as the
following:
```
function test() /*: $await<number> */ {}
```

Adding `f:/*::` skips comment continuations on new lines, when in a
type annotation comment. This needs to be prepended onto the existing
comments, to remain the behavior of javascript's block comments.