Support low-ceremony plugin development

[lacasseio]

The current plugin development process is pretty verbose. The "script plugin" concept helps with providing an easier way to write plugins. We could offer an event lower ceremony approach which would be really effective for injectable plugins as part of source dependency.

[lacasseio]

The low ceremony plugin development would allow the normal dev.gradleplugins.java-gradle-plugin and dev.gradleplugins.groovy-gradle-plugin to be applied in the settings.gradle[.kts] as settings plugins. It would configure the root project and disallow any nested projects. The sources would be found in the root project directory. This would enable creating a gradle plugin injectable into source dependencies using only 2 files.