Implement debug server

[allevato]

Once the basic UI to select and build targets goes in (#5), we'll have enough of a front-end in place that I can start moving my debugger prototype over.

[ittaiz]

@laurentlb you mentioned one can already debug starlark code in vscode. Is it in the open yet or will take some more time? Just had the need for debugging rules_scala and was hoping to skip print debugging :)

[allevato]

It's still a work in progress—I've checked in part of the implementation to add the UI to build targets with the debugger enabled, but it doesn't monitor thread state or stop on breakpoints yet.

Keep an eye on the repo—I'm hoping to have a usable debugging experience up here soon.

[ittaiz]

Thanks! Will do :)