Backspace at first LI should not merge with previous block

[fredck]

Starting HTML:

<p>Text in paragraph</p>
<ol>
    <li>^Item 1</li>
    <li>Item 2</li>
</ol>

Current results after Backspace:

<p>Text in paragraph^Item 1</p>
<ol>
    <li>Item 2</li>
</ol>

Expected results:

<p>Text in paragraph</p>
<p>^Item 1</p>
<ol>
    <li>Item 2</li>
</ol>

[Reinmar]

The same feature is required for the block quote feature.

[Reinmar]

To better describe what needs to be done:

• add listener to delete event on the view document,
• when in the first item of a list, outdent this item and prevent default.

The same needs to be done with the block quote feature.

Example of a custom Enter handling in the list feature: https://github.com/ckeditor/ckeditor5-list/blob/master/src/list.js#L52-L62.