Allow Evaluate code lens backend to send coverage data

[anderseknert]

While sprinkling some print calls here and there is often enlightening, having Regal (perhaps optionally) return coverage data from evaluation would allow clients to decorate evaluated expressions, similar to what's done in the VS Code OPA extension client side today, or in the Rego Playground.

[anderseknert]

So, this one turned out to be more... interesting than I had imagined. After setting up the plumbing to look into this, I was surprised to see that the coverage always came back as empty / zero for all values of covered / not covered. Turns out that the tracer that records coverage relies heavily on the File attribute having a set value on Location structs all over the AST. We have of course wiped that information since way back, as duplicating the same string on all AST nodes across an entire file is an enormous waste of resources. I think we're going to have to implement our own coverage tracer in order to make this work.