[1.x] Allow multiple previews for each pattern

Problem/Motivation

But I'm working with current version 1.7 and I have this specific need with a pattern referencing an image, and two variants.
In the first variant, I'm expecting the full image rendered from drupal with theme image.
In the second, I'm just expecting the URI of the image to set as a background.

So I have this need to define a different preview for a variant which I'm working on.

I'm aware of following existing issues, for incomming 2.x version.

https://www.drupal.org/project/ui_patterns/issues/3336654
https://www.drupal.org/project/ui_patterns/issues/3353289
https://www.drupal.org/project/ui_patterns/issues/3360373

Proposed resolution

Let's imagine this pattern

header:
  label: Header
  variants:
    rounded:
      label: Rounded
    color-1:
      label: Color 1
    color-2:
      label: Color 2
  fields:
    title:
      type: text
      label: Title
      preview: Hello, world!
    image:
      type: image
      label: Image
      preview: "//placehold.co/600x400"

I want a specific preview for the image field in rounded variant, and for demonstration purpose, I'll change the preview title too.

header:
  label: Header
  variants:
    rounded:
      label: Rounded
      preview:
        title: Hello, rounded!
        image:
            theme: image
            uri: "//placehold.co/200x200"
    color-1:
      label: Color 1
    color-2:
      label: Color 2
  fields:
    title:
      type: text
      label: Title
      preview: Hello, world!
    image:
      type: image
      label: Image
      preview: "//placehold.co/600x400"

Remaining tasks

Review and discuss

Data model changes

I added a preview attributes to PatternDefinitionVariant

  protected $definition = [
    'name' => NULL,
    'label' => NULL,
    'description' => NULL,
    'use' => NULL,
+    'preview' => NULL,
  ];