Improve reporting of the GitHub checks annotations

[uhafner]

Changes:

• Line coverage is now reported as consecutive line ranges.
• If a line is not covered (C0 = 0) then mutations are skipped.
• Mutator will be shown in the message of an annotation.
• Line and branch coverage annotations are skipped for mutation reports.

Fixes #775

[codecov[bot]]

Codecov Report

Merging #780 (e8b730d) into master (9804fdb) will increase coverage by 0.02%. The diff coverage is 95.45%.

@@             Coverage Diff              @@
##             master     #780      +/-   ##
============================================
+ Coverage     74.05%   74.08%   +0.02%     
- Complexity     1693     1698       +5     
============================================
  Files           130      130              
  Lines          6278     6289      +11     
  Branches        676      677       +1     
============================================
+ Hits           4649     4659      +10     
  Misses         1407     1407              
- Partials        222      223       +1     

Files	Coverage Δ
...overage/metrics/steps/CoverageChecksPublisher.java	93.00% <95.45%> (-0.10%)	:arrow_down:

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