[Demonstration Only] Cross-Version Support for Scala 2 and Scala 3

[srollins-lucid]

This is a proof of concept set of changes that demonstrates how to modify the scala-logging code to support projects that compile to both Scala 2 and Scala 3 at the same time. I do not want these changes (in their current form) to be merged into the scala-logging repo; in fact, I've rather gutted the project in order to make things work (for example, I had to remove LoggerTakingImplicits because cross-version macros didn't play nicely with implicits, and I dropped all of the scala-logging publishing stuff from the build since I didn't need it to build a local version that my project could depend on).

I was able to create a working cross-version solution that enables a project to migrate from Scala 2 to Scala 3 without being forced to do it all at once. I intend this code to serve as an example for anyone else who has the same need. This kind of solution should only ever be used on a temporary basis; after a project fully migrates to Scala 3, the public versions of the scala-logging library will be perfectly suitable for them again.

The solution to this cross-version problem was heavily inspired by the example mixed macros repo referenced in #317. There may be another solution that is not as destructive to the scala-logging library that could actually be merged into the repo, but I am not aware of what it would be.

[lightbend-cla-validator]

Hi @srollins-lucid,

Thank you for your contribution! We really value the time you've taken to put this together.

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