Shift-Tab in a list should demote the current list item

[MyriaCore]

If the mark is in a list of some sort, <backtab> should demote that list element.

- text - more text - child text - █

- text - more text - child text - █

If the bullet styles are different between the different levels, the demoted bullet type should be changed:

- text - more text * child text * █

- text - more text * child text - █

If the bullet types are different between the different levels, the demoted bullet type should be changed:

Caveats

For this to be usable, we should definitely make sure we parse list items correctly. Currently, any wite

- text - more text 1. child text 2. █

- text - more text 1. child text - █

If the bullet is at the last level, demotion should take it away from the bullet:

- text - more text - child text - █

- text - more text 1. child text █

This behavior might lead to typing mistakes, so it should be optional with a variable that controls for it.

Caveats

For this to be usable, we should make sure the parser recognizes same-indentation newlines as part of the list item. For example:

- List Item
- This list item is
  multiline. Everything I'm typing
  right now is still part of the list item.
- This list item has  multiple paragraphs.

  This paragraph is still part of the preceding
  list item. you can tall because the indentation
  of the list item is preserved.
- This list item has a code block as its second paragraph:
  
  ```
  there is code here
  it is part of the list item
  ```