Explore R6 for storing debugger state

As proposed by @sebffischer, R6 could be a good candidate for some of the internal state that gets passed around.

https://github.com/dgkf/debugadapter/blob/main/R/adapter.R#L22-L35

Of these, some are immutable and really don't need to be in a mutable state, but the `capabilities`, `breakpoints` and `breakpoint_id` would be candidates for R6 classes.

I'd be open to exploring an R6 class for either just breakpoints or the adapters/debuggers in their entirety. However, I'm inclined to still use a functional style preferentially, and only resort to R6's OO style when it fits particularly well - otherwise just using R6 for its mutable internal state.