expose API to get NimNode doc comments, for tooling

[timotheecour]

example use cases:

• in nim-docstrings @jboy has to resort to non-standard comments (via docstring"""foobar""" to expose them for his library:

However, I can't work out how to extract the text content of the documentation comment from my own code

(see also jboy/nim-docstrings#1)

• would allow this: option to dispatch from object's fields => leads to better API's and more DRY code option to dispatch from object's fields => leads to better API's and more DRY code c-blake/cligen#30
  which would enable the excellent project cligen from @c-blake to reuse documentation comments on object fields to auto generate help doc

• would allow for example nim --hints:list to show the doc comments for each hint

• would allow allow replacing this kind of PR https://github.com/nim-lang/Nim/pull/8625/files (Adds documentation for hints), (which is not DRY, and out of sync), with automatically generated doc comments.

EDIT
I have a PR that allows all the above, see #8903