Clear task names in Gradle plugin

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`.

Inspect the Pkl Gradle plugin's task registration and identify the code-generation tasks described in the issue. Run ./gradlew tasks to establish the current output; done when those tasks appear with a category and description that make them easy to locate and configure with finalizedBy.

Written by the indexing model from the issue text.