Added handling of Tab and Enter keys to the new Markdown editor, in line with what standalone text editors usually do. This is mostly focused on quickly writing and rearranging lists.

- Pressing Tab prepending 4 spaces to the line under cursor, or all the lines in the selection.
- Pressing Shift+Tab removes up to 4 spaces.
- Pressing Enter repeats any indentation and a "repeatable" prefix (list or blockquote) from the current line.
- To avoid interfering with keyboard navigation, the Tab presses are only handled once there has been some other interaction with the element after focusing.
- Pressing Escape removes focus from the editor and resumes default Tab navigation.
