Markdown toolbar: auto-continue ordered and unordered lists

[aitchiss]

The markdown toolbar currently has formatters for both ordered and unordered lists. Clicking the button will convert any highlighted text into a list by inserting - or e.g. 1. before each item. If no text is highlighted then an empty list is started.

We'd like to enhance this functionality by ensuring that as a user types their list, and presses Enter, we automatically insert the next line of list formatting.

Example: ordered list

Given the following text exists, with the cursor at the end of "Line 2":

1. Line one
2. Line two

If the user now types Enter, they should see:

1. Line one
2. Line two
3. 

And their cursor should be positioned after the 3.

Example: unordered list

Given the following text exists, with the cursor at the end of "Line 2":

- Line one
- Line two

If the user now types Enter, they should see:

- Line one
- Line two
- 

And their cursor should be positioned after the -

"Cancelling" the list formatting

We want an easy way for users to "escape" the list formatting - after we "auto insert" the next line of list formatting, if a user presses Enter again without adding any list item, we want to clear the formatting we inserted.

To continue the example above, given we have inserted 3. below:

1. Line one
2. Line two
3. 

If a user immediately presses Enter again, the 3. is cleared, leaving:

1. Line one
2. Line two

With the cursor remaining on the empty line under "Line two"

[github-actions]

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