Allow debugging custom rules

[anderseknert]

Being able to use the Debug code lens on custom rules would do a lot for the overall authoring experience. In theory, we'll only need to pass the parsed Regal bundle to the debugger for that to work, but a quick test shows some issues around bundle roots. This as the Regal bundle owns the "regal" root, and the workspace by default is also loaded as a bundle, and one that normally doesn't have a manifest pointing out any roots. Meaning it'll own all of them, leading to a conflict with the regal root.

Need to think more about how to best solve this. Suggestions welcome!