chore(lists): move list keymap to extra extension

[bdbch]

Please describe your changes

Because our custom keymap handling could potentially break when Prosemirror gets updated we decided to not have any custom fixes for that in our core but move it to an extra extension that can be loaded if the default keymap of Prosemirror is not sufficient.

This way we can also avoid breaking the core editing behaviour because of potential bugs in those custom fixes.

How did you accomplish your changes

Moved all of the keymap handling into it's own extension

How have you tested your changes

I created local demos - if you want to test it:

1. Check it out
2. Create a custom demo locally from taskList, bulletList or orderedlist
3. Add the ListKeymap extension
4. Check if the custom keymap handlers are triggered

Remarks

This is crucical to finally release a new stable version (2.1.0) as I don't want those keymaps in the core in 2.1.0

Checklist

• The changes are not breaking the editor
• Followed the guidelines
• Fixed linting issues

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