Add missing methods and fix inheritance in forms stubs

[emmanuel-ferdman]

PR Summary

This PR fixes forms module stubs to match runtime. It adds the missing __deepcopy__ methods to Field, ChoiceField, MultiValueField, and ModelChoiceField. It adds AltersData to the base classes of BaseModelForm and BaseModelFormSet so save_m2m is properly typed. It fixes BoundField.initial to use @cached_property instead of @property and adds __html__ to BoundWidget and Media. It corrects CheckboxSelectMultiple to inherit from RadioSelect like it does at runtime and moves id_for_label up to RadioSelect where it's actually defined. It removes phantom attributes that don't exist at runtime (HiddenInput.choices, RadioSelect.can_add_related, ErrorList.field_name) and fixes Input.input_type to be str | None since the base class has it as None. Also it exports ALL_FIELDS from django.forms, adds bound_field_class param to BaseForm.__init__, adds renderer param to formset_factory, and removes the bogus form_renderer/renderer params from BaseFormSet.__init__.

Final note: modelformset_factory and inlineformset_factory remain in the allowlist due to a stubtest false positive with type[TypeVar] params. I opened a PR to python/mypy#20950.