Use overload instead of override to implement Assertions.{assert, assume}

[ohze]

• Background
  Because dotty don't permit inline def override inline def, so in commit 3cfd618, in Assertions.{assert, assume}, we pass 'this to AssertionsMacro. And in AssertionsMacro, we switch the implementation logic of assert/ assume based on type of the passed param.

• Problem
  The solution above is not flexible enough if we want to extends Assertions as in scalatest/scalatestplus-junit project.

• Solution in this PR
  As suggested in Remove uses of inline overriding inline scala/scala3#8601 (review), I use overload instead of override to implement Assertions.{assert, assume}

[ohze]

Note: This PR must be merged before scalatestplus-junit can support dotty by scalatest/scalatestplus-junit#12

[cheeseng]

@ohze @bvenners Since 3.2.0 is around the corner, may I suggest we bring this dotty support to 3.2.0 instead of 3.1.x?

[cheeseng]

@ohze Fyi I have cherry-pick your commit(s) into this branch here for the upcoming 3.2.0:

https://github.com/scalatest/scalatest/tree/feature-3.2.0-with-dotty-improvements

[cheeseng]

@ohze Fyi changes included in this new PR for 3.2.0 #1827

[cheeseng]

@ohze Closing as this is now merged into 3.2.x-new and included in 3.2.0 release.