Support coverage reports

[esafak]

Make it possible to calculate what the test coverage is.

[vyadh]

Yeah, definitely want to get to this. I'm not 100% certain that Nushell has everything that is needed, but it has a lot. Try and see I guess.

[esafak]

Have your thoughts on the matter evolved since nushell/nushell#14227 ?

It seems the options are scripting w/debug profile, and hooking into nu-engine. If you are comfortable with Rust, and I'm not, I imagine the latter is the "right" way. Could we do it through a CoverageDebugger that satisfies debugger_trait?

[vyadh]

Sadly not very familiar with Rust, so I'd probably start by trying out the debug profile way, or at least until it becomes clear it's not going to work sufficiently for some reason. Having said that, I'm focusing on some work related stuff atm so not realistically likely to get to this soon though I'm sure it's going to be fun one to work on.