Included diagnostics in scip snapshot output

[matthewnitschke-wk]

This concept originally came out of this issue, which was individually reported in sourcegraph/scip within this issue

Adds the option to include indexed diagnostics to the results of scip snapshot

This functionality is explicitly enabled via the --include-diagnostics flag, to support backwards compatibility with current snapshot tests (eg, the inclusion of diagnostics in snapshot files is opt-in)

Example format is as follows:

• Given this example dart file

  void _unusedElement() {}

• Running scip snapshot --include-diagnostics will result in

     void _unusedElement() {}
  //      ^^^^^^^^^^^^^^ definition scip-dart pub test_package lib/main.dart _unusedElement().
  //      diagnostic UNUSED_ELEMENT

Test plan

• I've verified the functionality myself on various indexed scip files, but when attempting to write some unit tests around the new functionality, I was a bit confused on the current test setup, specifically around repolang and how to add diagnostic concepts to it. I'm open to suggestions / contributions to the pr addressing this missing test coverage

[jtibshirani]

@matthewnitschke-wk thank you for your contribution! I'm sorry we never responded to this, this PR fell through the cracks.

I wasn't able to push to your branch, so instead I opened #226. When it's merged, you'll be listed as a coauthor on the commit.

[matthewnitschke-wk]

@jtibshirani Awesome! Thanks for making those changes and polishing it a bit

I'll go ahead and close this pr then

[varungandhi-src]

Hey @matthewnitschke-wk apologies for dropping the ball here. If you have a PR that isn't reviewed within a few days, feel free to @ me.