ark: Debugger: Breakpoint support

[lionel-]

I haven't investigated how to support breakpoints yet. It's possible that we'll have to keep the debug session on at all times in order to add hooks that let us insert the relevant browser() calls. I noticed there is an option we might be able to use to hide the debug toolbar while R is not paused in the debugger, but I haven't explored that yet. Alternatively, there might exist a more direct VS Code API for hooking into the breakpoint UI. This would be ideal.

browser() insertions will need to happen behind the scene, using setBreakpoint() or following RStudio's approach.

[juliasilge]

This came up as a need during usability testing with @andrie, for R package development tasks.