Add a CommitMsg case class instead of using raw strings

[fthomas]

This replaces raw strings for the Git commit message with a CommitMsg
case class. Commit messages got more structure with the addition of
the Co-authored-by: trailer so it makes sense now to defer and move
the rendering of messages into a new class.

[codecov]

Codecov Report

Merging #2302 (1a95f0f) into master (5394a38) will increase coverage by 0.06%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master    #2302      +/-   ##
==========================================
+ Coverage   77.98%   78.04%   +0.06%     
==========================================
  Files         136      137       +1     
  Lines        2298     2300       +2     
  Branches       55       59       +4     
==========================================
+ Hits         1792     1795       +3     
+ Misses        506      505       -1     

Impacted Files	Coverage Δ
...ain/scala/org/scalasteward/core/edit/EditAlg.scala	64.51% <0.00%> (+2.01%)	⬆️
...rg/scalasteward/core/edit/hooks/HookExecutor.scala	93.10% <100.00%> (ø)
...steward/core/edit/scalafix/ScalafixMigration.scala	100.00% <100.00%> (ø)
...in/scala/org/scalasteward/core/git/CommitMsg.scala	100.00% <100.00%> (ø)
...n/scala/org/scalasteward/core/git/FileGitAlg.scala	98.27% <100.00%> (ø)
...in/scala/org/scalasteward/core/git/GenGitAlg.scala	92.30% <100.00%> (ø)
...main/scala/org/scalasteward/core/git/package.scala	100.00% <100.00%> (ø)
...calasteward/core/vcs/data/NewPullRequestData.scala	100.00% <100.00%> (ø)

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