Add support for multiple cursors

We should add support for multiple cursors because they're widely used and useful. Default shortcuts should be:
* Ctrl+Alt+Up/Down for duplicating the cursor above/below
* Ctrl+D for finding the next occurrence of the current selection and adding a cursor there

This requires:
* Extending the history entry: https://github.com/microsoft/edit/blob/b008570afc0c4c6cef7ba0335d407139a47c8e24/src/buffer/mod.rs#L84-L101
* Splitting the singular cursor into a main cursor and a secondary cursor list (which could also include the main cursor?): https://github.com/microsoft/edit/blob/b008570afc0c4c6cef7ba0335d407139a47c8e24/src/buffer/mod.rs#L186
* Centralizing / abstracting the various cursor movement functions within the same file
* ...and making all cursors move when the main cursor moves
* Updating `write()`, `delete()` and `unindent()` accordingly.

Start in src/buffer/mod.rs at the history entry and cursor definitions referenced in the issue, then trace the cursor movement functions and the write(), delete(), and unindent() paths. Done means supporting the listed Ctrl+Alt+Up/Down and Ctrl+D shortcuts, recording multiple cursors in history, and applying movement and editing operations to all cursors.

Written by the indexing model from the issue text.