Log4j2 compatibility

## Context

https://github.com/clojure-emacs/cider-nrepl/pull/773#discussion_r1226611259

## Brief

> Right now this works with Logback (which I am using) and Java Util Logging. There is some code for Log4j2 that passes some of the tests, but I couldn't make it working reliably. The issue seems that Log4j2 reloads its configuration when a new appender is added, and wipes out the changes. I tried most of the suggestions on Stackoverflow to get it working, but I'm giving up on this one. I already spent too much time on this. So, I propose to drop that code into a branch of the separated library. If anyone knows how to get it working, I'm willing to help, but I won't investigate this beast any further.

# Possible solutions

* Persevere through the original approach
* Try a less dynamic approach:

>> Would it help if the nrepl logger wasn't dynamically added?
>> e.g. one could configure it such that it's instantiated as the JVM starts.
>> This would probably involve a .xml file or a user.clj file.
>> ...this would not invalidate the current, dynamic approach, I'd see it instead of an additional, opt-in manner to run things.

> It might, but I believe for this to work we would need to use gen-class to create an appender class that implements some Log4j2 interface and ship it, so that it can be referenced in the configuration file.
> And then somehow connect this with the atom to which events are added.