[6.x] Add support for custom link types

[duncanmcclean]

This pull request implements support for registering custom link types on the Link fieldtype, and unifies Bard's link button to use the same system.

Previously, the Link fieldtype only supported linking to a URL, Entry, Asset, or (in a collection with a structure) the first child page. There was no way for addons, like Runway and Forms Pro to add their own link types.

Addons can now register a link type from their service provider:

use Statamic\Fieldtypes\Link;

Link::extend('form', \App\FormLinkType::class);

A LinkType controls how the type is displayed, how a stored value resolves back to a real link, and what fieldtype powers its picker:

use Statamic\Facades\Form;
use Statamic\Fields\Field;
use Statamic\Fieldtypes\Link\LinkType;

class FormLinkType extends LinkType
{
    public function resolve(string $id, $parent = null, bool $localize = false): mixed
    {
        $form = Form::find($id);

        if (! $form) {
            return;
        }

        return "/forms/{$form->handle()}";
    }

    public function fieldtype(Field $field): ?array
    {
        return ['type' => 'form', 'max_items' => 1];
    }
}

Once registered, the Link fieldtype will show the link type in the dropdown and render a picker using the provided fieldtype.

This PR also migrates the native Entry and Asset options onto the same LinkType system, so there's a single code path for native and third-party types alike.

Bard

Bard's link button previously had its own, entirely separate implementation for resolving entry::/asset:: references (via statamic:// URIs and the Data repository) and its own hardcoded Entry/Asset picker.

This PR unifies it onto the same Link::extend() registry — any custom link type registered for the Link fieldtype is now automatically available in Bard's link button too, with no extra wiring needed.

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Closes statamic/ideas#1181
Docs: statamic/docs#1949