Effective immediately! Drop support for stable R :stuck_out_tongue_closed_eyes:

This project hit a few constraints that made it mostly dead-on-arrival. Most notably, the lack of any hooks for interacting with an ongoing browser session.

Well that looks like it's unofficially been solved in [bugs.r-project.org discussion](https://bugs.r-project.org/show_bug.cgi?id=18590).

We now have an experimental global option to add a browser "error handler", which allows us to hook into conditions intercepted by the browser.

I see this as almost an inevitability in the R world, so I'm willing to stake the project against the eventual inclusion of such a feature. Instead of even entertaining support for current versions of R and contorting the project to work in that scheme, I think it's better to throw all that out and use this much more direct mechanism of intercepting the debugger.