Eval: add "Evaluate rule" (as opposed to document) code lens

[anderseknert]

It would be really useful if Regal allowed evaluating both whole documents (like we currently do) and individual rule definitions. So if there are e.g. 3 allow definitions, we would present 2 code lenses for each of them:

• Evaluate document
• Evaluate rule

The "Evaluate document" functionality is of course already in place. I imagine the easiest way to allow eval of a single definition is simply to strip out the rest of the rules with that name from the AST before sending it for eval.