Support `mill init` from existing Gradle project (1500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 1500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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Same as #3449 and #3450 but for Gradle instead of Maven and SBT.

[lihaoyi]

CC @ajaychandran in case you're interested, since you're already familiar with all the code and much of it can be shared

[lihaoyi]

Bumping the bounty from 1000->1500USD

[ShreckYe]

This is a bit of a disappointment for me as I've also been working on this for a while and looks like @ajaychandran has beaten me to it 😅. Congratulations Ajay! I hadn't seen any updates here in a while, and mistakenly assumed no one is working on it. So lesson learned – I should have commented and asked. I've encountered some points during my own implementation, so I'd like to join the code review and share some suggestions if I may.

[lihaoyi]

@ShreckYe please go ahead and join the code review!

[lihaoyi]

@ShreckYe in fact I'll happily pay you 500USD, separate from the bounty, just to give a rigorous code review of @ajaychandran's PR given your experience wrestling with the problem in your own fork

[ShreckYe]

Thanks @lihaoyi. I will do it now.