Add ability to persist latest value between runs

In getting line AA working, I've often been dragging the start of a line (defined by an option in the debug tools) to a particular place to either produce an error, debug an attempt to fix that error or to see the error has gone. When iterating on features, it gets old pretty fast to have to note down the failing coordinates and drag the line to that position again on restart of the debug tool.

It would be super useful to be able to configure certain options (or a more coarse grained all/none, I don't mind) to persist into a file between runs of the debug tools to be able to quickly start up with the previous parameters again.

I was thinking of a very simple JSON file with the same name as the binary being run, stored next to it but impl is an open question.

I think this leads to problem with e.g. `Point` and `Size` requiring ser/de impls which, from what I can see, would require a `serde` feature added to e-g with another `cfg` option as `defmt` already has. Maybe this is a good thing, but it's more noise in e-g which I don't feel great about.