Redo debugging (and add support for Debug gem)

[Tabby]

I started looking into making similar changes as in PR #118 in the main repo and I came to the conclusion that the approach we're taking to handle debug configuration is probably wrong.

While it definitely makes it easier for users to get debugging working by making our own DebugConfigurations, it means that we either have to add a ton of config options to allow customisation of the debug integration, or just not let users configure it. Neither of those seem like great choices.

The thing that particularly got me thinking about this, is that in the above PR the DebugConfiguration for the Debug gem has localfs: true set in it. This is probably the right choice for the majority of users, but seems like it won't be correct for anyone who needs remote debugging and we'd have to add another config option to allow people to set it.

What I think we should be doing instead is making clear documentation, with examples, on how users can write DebugConfiguration files in their projects and then only having configuration options to set the name of the configurations to use, and possibly which debugging gem to use and the command to run it.

We could optionally try to generate default entries in the launch.json file to help people get started.

This way, we have less work to do, to handle the configuration and generate debug configurations and so on, and users are able to configure debugging in the standard VSCode way (via launch.json)

It's also likely that the code for handling debugging in general can be refactored and/or simplified/improved a lot. I'm not sure it's even currently working at all, and needs testing in a local environment >_>