Auto-filling succeeding numbers on numbered lists

### What
With Markdown enabled, it would be great to have the ability to easily create numbered lists by autofilling succeeding numbers when pressing the Enter key.

To illustrate: when I type 1. and hit Enter, 2. automatically appears, then 3., so on and so forth. At the moment, the autofilling only works for unordered lists.

### Why
This already exists for bulleted lists and it is great!

Start by locating the existing unordered-list autofill handling for the Enter key in the Markdown editor. Compare its behavior with numbered-list input, then verify that pressing Enter after a numbered item inserts the next number and that the sequence continues as described.

Written by the indexing model from the issue text.