Mill init support

[vaslabs]

This is an overall discussion to discuss and plan mill init support, specifically for the major jvm frameworks (SpringBoot, Micronaut and Quarkus). Later on we should add support for Kotlin and Android too.

Brief summary

The plan I propose (which is already underway) is:

• Support basic Java setups for all 3 frameworks (at the point of writing SpringBoot is done, Quarkus PR) . The new basic setup will use the new Spring Boot, Quarkus and Micronaut modules.
• Then we dig into details and extensive support, starting with SpringBoot as the most popular framework (examples to use as shown in Make ./mill init work for spring-boot-examples project #6077 )

Once spring boot is done, we need to find a similar examples repo for the other two frameworks.

Kotlin

• For kotlin, we can start with mixed SpringBoot and Micronaut setups and get any missing kotlin support out there.

Android

• Once kotlin is done, we can proceed to supporting android. We should discuss further details once we get there.

@souvlakias, @lihaoyi wdyt?