add gitlab_output(): a report format digestible by gitlab

[lschneiderbauer]

Addresses #2858.

I tested on gitlab whether gitlab compains about the format by pushing a locally generated report: https://gitlab.com/lschneiderbauer-group/lintr-test/-/merge_requests/1
It seems to work fine:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (56c3c6d) to head (9b7e2c9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2862   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files         127      127           
  Lines        7080     7109   +29     
=======================================
+ Hits         7055     7084   +29     
  Misses         25       25           

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[lschneiderbauer]

@MichaelChirico I would appreciate your feedback.

[MichaelChirico]

High-level LGTM. I'll take your word for it that the integration is WAI.

I do wonder if maybe we should be extending print() instead (or in addition to?) adding new exports, e.g. print(output = "gitlab"), print(output = "sarif"), etc. But that's an issue for another day I think.

Please add a NEWS entry.

[lschneiderbauer]

I'll take your word for it that the integration is WAI.

I plan to use that feature quite regularily within our CI pipeline. In case any broken edge cases come up, I will submit a fix.

[MichaelChirico]

Thanks!!