[5.x] Add recursive form:fields tag with support for group fieldtype

[jesseleite]

Right now, if you Group::makeSelectableInForms(), a bunch of things break because there is no payload handling for recursion in the {{ fields }} loop.

TODO:

• Recursively prepare renderable field objects
  • Non-JS forms
    • Make sure group children submits correct name (ie. some[child][field])
    • Make sure error bag still works
    • Make sure old input still works
  • Alpine driver forms
    • Make sure group children submits correct name (ie. some[child][field])
    • Make sure error bag still works
    • Make sure old input still works
• Flesh out default group.antlers.html view to be used by {{ field }} rendering within {{ fields }} loop
• Change other inputs to use name instead of handle (because they need to be nested[field][format])
• Add test coverage
• Make group fieldtype available to frontend forms

Closes statamic/ideas#1228