Integrate ./mill init with the example projects (500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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Mill's example/ folder projects are all zipped up and published for download with every release, and form a far more comprehensive and richly documented set of project templates than is available on giter8. We should update ./mill init to prioritize using these examples as getting started templates for users using ./mill init. We can continue to support giter8 if we want, but I expect the built in example templates will be what people want 99% of the time and render external giter8 templates largely unnecessary.

Success criteria:

• When you run ./mill init without any argument, it shows the user a list of all the Mill example/ projects, grouped and organized in some meaningful way
• The user can then select one of those example projects (e.g. by typing in its number in the list, or its path e.g. javalib/basic/1-simple) and Mill will download the project from the Mill Releases page and unpack it into the local directory
• The user can also pass in the path of the example as an argument, and as long as it matches one of the accepted example paths, Mill will download and unpack the example zip rather than use giter8

The list of valid examples can vary between Mill versions, and should be computed at build time as a resource generator and bundled together with Mill during creation of its dist.assembly.

[pawelsadlo]

does it need to run interactively, prompting example number just like init is working now?
or its ok to ask user to run ./mill init [number or path of example]

[lihaoyi]

@pawelsadlo no need interactivly for now, AFAIK mill init today is non interactive:

$ ./mill init
Creating a new project...
Error: Missing argument <template>
Try --help for more information.

I think it would be fine just to print out a big list of potential templates and then ask the user to copy paste the one they want for now. Once all the example templates are all wired up and in place, we can separately look at tweaking the CLI experience

[pawelsadlo]

Ah right , the g8 template generation is interactive, not the init command.
Anyway can mill command even be interactive without running creating a subprocess?

[lihaoyi]

./mill -i does allow interactive stuff, like console. Even without a subprocess you can do the same thing and read/write from stdin/stdout

[lefou]

For the purpose of copy-and-paste in tutorials and readmes, it would be nice when mill init can be used non-interactively.

[llvee]

@pawelsadlo
Did you complete this? If not maybe I will attempt this in 24 hours from now.

[pawelsadlo]

@llvee
it's done ,I'm just adding some tests

[llvee]

@pawelsadlo let me know if you'd like some help with test writing.

[lefou]

I think accepting a --mill-version option to choose the version from which we take the examples from might be a good idea. This can be helpful in cases where the Mill version isn't exactly the one providing the wanted examples.