Add support for multiple cursors

[lhecker]

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:

  edit/src/buffer/mod.rs

  Lines 84 to 101 in b008570

  	/// An undo/redo entry.
  	struct HistoryEntry {
  	/// [`TextBuffer::cursor`] position before the change was made.
  	cursor_before: Point,
  	/// [`TextBuffer::selection`] before the change was made.
  	selection_before: Option<TextBufferSelection>,
  	/// [`TextBuffer::stats`] before the change was made.
  	stats_before: TextBufferStatistics,
  	/// [`GapBuffer::generation`] before the change was made.
  	generation_before: u32,
  	/// Logical cursor position where the change took place.
  	/// The position is at the start of the changed range.
  	cursor: Point,
  	/// Text that was deleted from the buffer.
  	deleted: Vec<u8>,
  	/// Text that was added to the buffer.
  	added: Vec<u8>,
  	}

• Splitting the singular cursor into a main cursor and a secondary cursor list (which could also include the main cursor?):

  edit/src/buffer/mod.rs

  Line 186 in b008570

  cursor: Cursor,

• 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.