Switch to scales::Range classes

[teunbrand]

This PR aims to fix #2710.

In brief, it replaces the unexported Range ggproto classes with the scales::Range R6 classes. This makes it slightly easier for extension developers to use the Range systems. Extenders have been able to do this for a while, but hopefully this serves as an 'endorsement' of using scales' Range classes.

[yutannihilation]

Looks good to me, but I'm not fully sure if there's no package that expects a range object is a ggproto, so let me keep this unmerged for a while.

[teunbrand]

Yeah good point. It seems like a good idea to search github for the use of ggplot2:::continuous_range()/ggplot2:::discrete_range() and inform people that they might consider to switch to the scales::Range classes.

[teunbrand]

Moreover, this move would be a little bit too risky for an upcoming patch release, so I think it'd be best to wait for after that has happened. I'll convert this to a draft until that happens, so it isn't accidentally merged.

[yutannihilation]

I agree with you. Thanks, let's wait.

[teunbrand]

Hi @yutannihilation, with ggplot2 3.4.1 having landed on CRAN, do you think this is OK to merge now?

[yutannihilation]

Oh, thanks for the notice. I think so, but maybe we can merge #5171 first? We'll anyway need to resolve conflict on NEWS.md there, so I think either is fine.

[yutannihilation]

Now that #5171 got merged, let's merge this.

[teunbrand]

Thanks Hiroaki!