Make NuGet packages installable with "dotnet tool"

[zooba]

Feature or enhancement

Reference (not how we'll do it, but a starting point): https://learn.microsoft.com/en-us/dotnet/core/tools/global-tools-how-to-create

We ought to be able to make dotnet tool install python work with some metadata in our NuGet package. We should do this, because it'd be awesome.

[zooba]

Turns out we could only do this with a .NET assembly (it only supports .DLLs), and while it would be possible to add one that launches our EXE, it's probably not really worth it.

If PEP 773 gets rejected then I'll probably revisit this, but I'd rather just fix our own stuff before investing heavily in other ecosystems.