Add authors field to ScalafixMigration

[fthomas]

Closes #1914.

[codecov]

Codecov Report

Merging #2301 (267c42e) into master (d570d75) will increase coverage by 0.06%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #2301      +/-   ##
==========================================
+ Coverage   77.89%   77.96%   +0.06%     
==========================================
  Files         135      136       +1     
  Lines        2289     2296       +7     
  Branches       52       53       +1     
==========================================
+ Hits         1783     1790       +7     
  Misses        506      506              

Impacted Files	Coverage Δ
...ain/scala/org/scalasteward/core/edit/EditAlg.scala	62.50% <0.00%> (+1.89%)	⬆️
.../main/scala/org/scalasteward/core/git/Author.scala	75.00% <75.00%> (ø)
...steward/core/edit/scalafix/ScalafixMigration.scala	100.00% <100.00%> (ø)

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