Custom Block behaviour for create and delete

[jstrobel]

Describe the bug

1. Position of new Block is not "after" - it is inserted before the custom field.
2. Deleting removes first of all Blocks

To Reproduce Steps to reproduce the behavior:

1. https://github.com/getkirby/editor/wiki/Block-extensions Example to your Kirby-CMS Installation
2. Add the new "Custom" Block
3. Add a new Block e.g. Text "under" the custom Block
4. Block is created above

When you try to delete the custom Block the first Block of all is deleted.

Kirby Version 3.5.1