Redesign `[debug]` directive for end user opt-in

The `[debug]` directive allowed for a consistent way to attach a debugger and stop on a breakpoint in an application's startup code. We weren't satisfied with the security of the design. Ideally, this functionality should be opt-in by the end user rather than determined by the application developer.

One proposal is to have the debug check verify the presence of a different user-installed tool before pausing to attach the debugger.