Add ability to set custom placeholder in lists

[ocean90]

Following #1891 for text and headings, this adds placeholder as an attribute to core/list to make the placeholder customizable when the block is used in a template.

[gziolo]

Technically it would work, but it will cause that the placeholder text would be stored in HTML comments for every List block. We should seek an alternative solution which will allow to set this value in the template, but wan't be part of attributes definition. Maybe we can use supports definition instead and make placeholders customizable for all blocks.

[ZebulanStanphill]

If the List block started using nesting and a List Item child block was added (see #6394), then would that make this easier?

[danielbachhuber]

Closing, per:

We should seek an alternative solution which will allow to set this value in the template