This PR initially started out as an attempt to add group field support, but has grown to include a whole bunch of improvements, with the purpose of improving dynamic field rendering for users, and to prepare for our upcoming GUI based form builder stuff.

TODO:

• Pass both handle and name into fields context
  • Because nested fields need to submit nested[field][format] input names
  • But we still need dotted nested.field.format handles for showField() JS driver logic
• Add support for group field
  • Make group fieldtype available to frontend forms
  • Flesh out default group.antlers.html fieldtype template
  • Change individual fieldtype views to submit using new name[format] instead of handle.format
  • Add preProcessTagRenderable() to fieldtypes
  • Update trait to call preProcessTagRenderable() for group-specific recursion logic
  • Make sure error bag still works
  • Make sure old input still works
  • Make sure JS conditions work on nested fields
    • Updates alpine driver
    • Update abstract JS driver so that custom drivers can easily render attributes onto group fields
  • Make sure JS conditions work on parent group field itself
    • Hidden fields should hide all children
  • Make sure show_fields array still works at top level for hardcoded fields outside of fields loop
    • Update documentation to show recommended syntax for this (ie. show_field['nested.dotted.handle'])
  • Fix old value bug when submitting partial nested field payloads
• Add {{ form:fields }} recursive tag helper
  • Make sure {{ form:fields }} works recursively using {{ fields }} array context at every level
  • Make sure {{ form:fields }} works recursively within {{ sections }} loop as well
  • Support scope param (ie. {{ form:fields scope="field" }})
• Add support for recently added $parent conditions syntax, since we assume feature parity with CP
  • See: [5.x] Add parent keyword to field conditions #9385
• Fix checkbox null filtering recursively once and for all
  • See: [4.x] Filter out any nulls from front end form checkboxes #9212
  • Breaking: Support for null values on things like checkboxes, radios, selects, button groups, etc. in the CP GUI was technically dropped in 4.56.0. This PR removes some extra/related backend handling, so if you still have this in your yaml, it could be breaking for some people in v6.
    • See: [4.x] Improve array fieldtype validation for dynamically keyed fields #9834
• Improve field html id rendering
• Test coverage
• Documentation
  • Document new form:fields changes docs#1606

Closes statamic/ideas#1228