…ting Gradle project (com-lihaoyi#3962) referencing com-lihaoyi#3756

- Add an `InitFromGradleModule` object and the trait with the same name, with common logic extracted from the `InitMavenModule` trait into the `InitFromAnotherBuildToolModule` trait
- Add necessary dependencies and repository to use the Gradle Tooling API
- Add lines in `MainModule.init` for Gradle adapted from those for Maven
- Copy and adapt `BuildGen` and `BuildGenConfig` for Gradle from the `maven` module
- Initially configure the `GradleConnector` and retrieve the `GradleBuild` with the Gradle Tooling API in `mill.main.gradle.BuildGen`
- extract and move common build generator logic into the `build-gen` module and the `buildgen` package

  A `CommonBuildGenConfig` is extracted for both Maven and Gradle. "ir.scala" and "Tree.scala" and moved here from the `maven` module.

  As the original `build` package name is somewhat too broad and its directory is ignored by .gitignore by default, I don't think it's a proper name to use here.