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