Include diagnostics in snapshot output

[jtibshirani]

This change adds diagnostic information to the snapshot output to support
testing. To make it possible to test this in the SCIP repo, it updates the
reprolang generator to output a diagnostic for every identifier that starts
with 'deprecated'.

Example output:

 reference deprecatedMethod.
#          ^^^^^^^^^^^^^^^^^ reference diagnostics.repro/deprecatedMethod.
#          diagnostic Warning: deprecated identifier

Closes #213

Test plan

New snapshot test to check diagnostics output

[jtibshirani]

This PR is based on #220, with the following differences:

• Don't introduce a new flag --include-diagnostics, as I think it's fine to just enable this by default. When people update to this new SCIP version, they can just update their test snapshots if needed.
• Print the diagnostic warning and message instead of the 'code', as these are required fields and more likely to be present

[varungandhi-src]

Nice work. I noticed that we were truncating the multiline output for docs (and you probably copied that for the diagnostic message), which seems like undesirable behavior from the POV of an indexer as you want to make sure the full contents are correct. So I've tweaked that.

[varungandhi-src]

Rebased on include the build failure fix from #228