[6.x] Forms 2: Add appendConfigFields to form fieldtypes

[duncanmcclean]

This pull request adds a way for addons to append config fields to form fieldtypes.

Form fields use dedicated form fieldtypes (eg. Short Answer, Dropdown) which wrap a regular fieldtype under the hood (Short Answer wraps text).

These build their config fields from their own curated configFieldItems(), so addons appending config fields to a regular fieldtype via Fieldtype::appendConfigField() never showed up in the Form Builder.

FormFieldtype now has its own appendConfigFields() and appendConfigField() methods, mirroring the existing Fieldtype API. You can append to a specific form fieldtype, or to every form fieldtype by calling it on the FormFieldtype base class:

use Statamic\Forms\Fieldtypes\ShortAnswer;

ShortAnswer::appendConfigField('character_limit', [
    'display' => 'Character Limit',
    'type' => 'integer',
]);

Backwards compatibility

This PR also bridges config fields appended to a wrapped fieldtype (eg. Text::appendConfigField()) onto the form fieldtypes that wrap it, so addons written against the old API keep working without changes. If both APIs append a field with the same handle, the form fieldtype's own append wins.

This "bridge" is temporary and marked with a TODO to be removed in v7. Appending to the wrapped fieldtype is indirect — the addon is really targeting text fields everywhere (entries, globals, etc.), and only reaches forms because form fieldtypes happen to wrap regular fieldtypes as an implementation detail.

The bridge exists purely because Forms 2 ships in a minor release where existing addons can't be broken. By v7, addons should append to form fieldtypes directly using the new API.