Introduce EditAttempt enum

[fthomas]

This introduces the EditAttempt enum for the different types of edits Scala Steward can do in a PR: version bumps, Scalafix changes, and post-update hooks changes. The purpose of this change is that we can in the future use the list of EditAttempts returned by EditAlg to create more faithful PR bodies that better reflect what Scala Steward did: for example only mention Scalafix migrations if they result in changes, indicate if a Scalafix migration failed, or add labels for post-update hooks., etc.

[codecov]

Codecov Report

Merging #2242 (5b3e881) into master (4c9c1a0) will increase coverage by 0.08%.
The diff coverage is 57.89%.

@@            Coverage Diff             @@
##           master    #2242      +/-   ##
==========================================
+ Coverage   78.23%   78.31%   +0.08%     
==========================================
  Files         131      132       +1     
  Lines        2265     2283      +18     
  Branches       48       57       +9     
==========================================
+ Hits         1772     1788      +16     
- Misses        493      495       +2     

Impacted Files	Coverage Δ
...scala/org/scalasteward/core/edit/EditAttempt.scala	0.00% <0.00%> (ø)
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0.00% <0.00%> (ø)
...ain/scala/org/scalasteward/core/edit/EditAlg.scala	59.37% <75.00%> (+0.75%)	⬆️
...rg/scalasteward/core/edit/hooks/HookExecutor.scala	93.10% <100.00%> (ø)
...calasteward/core/vcs/data/NewPullRequestData.scala	100.00% <100.00%> (ø)
.../main/scala/org/scalasteward/core/io/package.scala	100.00% <0.00%> (ø)
...scala/org/scalasteward/core/scalafmt/package.scala	100.00% <0.00%> (ø)
...a/org/scalasteward/core/edit/UpdateHeuristic.scala	98.87% <0.00%> (+0.03%)	⬆️
...org/scalasteward/core/buildtool/mill/MillAlg.scala	86.95% <0.00%> (+0.59%)	⬆️
... and 1 more

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[fthomas]

That we can get rid of ScalafixMigrationsFinder in NurtureAlg is what I like most about this change.