Comments as entities

Right now Multiplier doesn't actually understand comments. I think it'd be nice to make it understand comments in the code, and then also make user annotations a variant of this, e.g. there could be an entity class hierarchy like:

- `Comment`
  - `CodeComment`
    - `MultiLineComment`
    - `SingleLineComment`
  - `UserComment` or `FloatingComment` or `Annotation`.
