Move code generation to `log4j-codegen` artifact

The code generation code is moved from `log4j-core` to a new
`log4j-codegen` module. Additionally:

* The embedded PicoCLI copy is replaced with an external PicoCLI dependency.
* `log4j-codegen` publishes a shaded artifact with the classifier
  `shaded`.
* We add a documentation page with basic usage.
14 files changed