feat: support named exports for wrapper components

[mattjennings]

This adds support for a custom Wrapper when the component is a named export.

Via remark config:

{
  remarkPlugins: [
    [
      examples,
      {
        defaults: {
          Wrapper: ['some-package', 'CustomExample'] // -> import { CustomExample } from 'some-package'
        }
      }
    ]
  ]
}

Via inline config:

```svelte example Wrapper=["some-package", "CustomExample"]

```