Support basic expression evaluation in debug session

In https://github.com/open-policy-agent/regal/pull/2051 I have tried an experimental (and somewhat hacky) implementation of the Evaluate feature from the DAP specification. This essentially fakes an evaluation in the debug session by prepending the expression query with the variables in scope of a frame. Having OPA's debugger itself being able to do this would make that less brittle, and without depending on unsafe hacks. Evaluating expressions in the context of a session/frame is also a prerequisite for supporting conditional breakpoints.