Support multiple default resolvers

[fthomas]

The default resolver - which is Maven Central if none is provided by the --default-maven-repo option - is now used in multiple places where a list of resolvers is expected. It feels like an arbitrary restriction that we only allow one resolver and always wrap that in a list everywhere we use it. Why not allow --default-maven-repo to be a repeating option instead so that Config has a List[Resolver]. This PR does exactly that. It could be useful if a user wants to resolve Scalafmt via resolver A and Gradle dependencies via resolver B for example.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.79%. Comparing base (af9dad6) to head (958820e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../scala/org/scalasteward/core/application/Cli.scala	80.00%	1 Missing ⚠️
...a/org/scalasteward/core/buildtool/sbt/SbtAlg.scala	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3570   +/-   ##
=======================================
  Coverage   89.79%   89.79%           
=======================================
  Files         174      174           
  Lines        5028     5029    +1     
  Branches      502      491   -11     
=======================================
+ Hits         4515     4516    +1     
  Misses        513      513           

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[exoego]

LGTM