Import MockEffOps to fix compile errors with Scala 3

[fthomas]

This is in preparation for compiling with Scala 3 and fixes errors like this:

 -- [E008] Not Found Error: modules/core/src/test/scala/org/scalasteward/core/BuiltinConfigFilesTest.scala:38:9
 36 |      val migrations = artifactMigrationsLoader
 37 |        .loadAll(ArtifactCfg(List(), disableDefaults = false))
 38 |        .runA(initialState)
    |                       ^
    |value runA is not a member of org.scalasteward.core.mock.MockEff[
    |  List[org.scalasteward.core.update.artifact.ArtifactChange]], but could be made available as an extension method.
    |
    |The following import might fix the problem:
    |
    |  import org.scalasteward.core.mock.MockEffOps

I think the change in the Scala 3 compiler that makes this import necessary, is "3. Package prefixes no longer contribute to the implicit search scope of a type".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.73%. Comparing base (e869714) to head (4ad0198).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3519   +/-   ##
=======================================
  Coverage   89.73%   89.73%           
=======================================
  Files         171      171           
  Lines        3536     3536           
  Branches      321      321           
=======================================
  Hits         3173     3173           
  Misses        363      363           

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[mzuehlke]

Because #2065 isn't merged, yet.
CI only tests Scala 2.13 and not Scala 3.
I guess you verified this locally.

[fthomas]

I cherry-picked this commit from the topic/scala3 branch, so it should be fine. My idea is to break down #2065 into smaller PRs that are uncontroversial, easy to digest, and are about just one topic.