[base] New form field components

[mariuslundgard]

Type of change (check at least one)

• New feature (non-breaking change which adds functionality)

Does this change require a documentation update? (Check one)

• No (but it will soon)

Description

This change introduces 4 new public components for building form field components:

import {
  FormField,
  FormFieldHeaderText,
  FormFieldSet,
  FormFieldValidationStatus,
} from '@sanity/base/components'

Note for release

• New building blocks for composing form fields, that will be used by @sanity/form-builder and to create custom input components.

[shortcut-integration]

This pull request has been linked to Clubhouse Story #1502: [base] New FormField components.

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[bjoerge]

Left a few questions about a couple of minor things that was a bit unclear to me while reading through. Otherwise LGTM!