Add adaptive_breakpoints Based Gap subclasses

[caseycrogers]

I built the following package that is bscly a shittier version of Gap, except that it also provides Gap style classes that automatically size themselves according to adaptive_breakpoints so you don't have to specify the size manually:
https://pub.dev/packages/flutter_gutter

Honestly, this doesn't really need to be it's own package. It'd probably be way more useful to devs if this functionality were rolled into Gap. What do you think?

If you're for it, I'd be happy at making a first-pass PR to migrate my package's functionality into your package.