Replace case-app with decline

[fthomas]

This replaces case-app with decline for command-line arguments parsing. My main motivation for doing this was to get familiar with decline but it also has some benefits:

• Help messages are ordinary values instead of annotations of fields. This means we can use variables and string interpolation in those messages which is currently not possible in annotation arguments. The biggest benefit here is that we don't need to repeat default values verbatim.
• Relationships between different options can be expressed. For example, --enable-sandbox and --disable-sandbox are now mutually exclusive and it is a runtime error if both are specified. Similar, if one of the GitHub app option is specified, the other must be specified too and not doing so is again a runtime error.
• decline does not use macros and therefore is already available for Scala 3:
  • https://repo1.maven.org/maven2/com/github/alexarchambault/
  • https://repo1.maven.org/maven2/com/monovore/decline_3/

The only downside I'm seeing so far is that defining the options is a little bit more work than than writing the Args case class.

[codecov]

Codecov Report

Merging #2414 (b0b8d2f) into master (2595822) will increase coverage by 0.50%.
The diff coverage is 97.45%.

@@            Coverage Diff             @@
##           master    #2414      +/-   ##
==========================================
+ Coverage   80.31%   80.81%   +0.50%     
==========================================
  Files         141      141              
  Lines        2393     2445      +52     
  Branches       53       53              
==========================================
+ Hits         1922     1976      +54     
+ Misses        471      469       -2     

Impacted Files	Coverage Δ
...re/src/main/scala/org/scalasteward/core/Main.scala	0.00% <0.00%> (ø)
...ala/org/scalasteward/core/application/Config.scala	100.00% <ø> (+2.32%)	⬆️
...la/org/scalasteward/core/application/Context.scala	70.58% <ø> (+1.02%)	⬆️
...main/scala/org/scalasteward/core/vcs/VCSType.scala	100.00% <ø> (ø)
.../scala/org/scalasteward/core/application/Cli.scala	100.00% <100.00%> (ø)
...in/scala/org/scalasteward/core/util/dateTime.scala	100.00% <100.00%> (ø)

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