Clear task names in Gradle plugin

[sgammon]

It is currently pretty hard to locate the task names for generating code from the Pkl Gradle plugin; it would help if these task names were given a category and description, so that they show up when a Pkl user runs ./gradlew tasks.

In my specific use case, I'm looking for the tasks I can configure to run another task reliably after Pkl's code-gen step. To remedy #187 locally, I want to amend the files and move them to a package path, but it's hard to do that even with Gradle's extensive override facilities if I don't know where plugin authors prefer me (as a build author) to hook in and call finalizedBy.