WIP: more multiple-cursor commands

[Book-reader]

this pr adds more commands to multiple-cursors.lisp that emulates the mark-next/previous-like-this functionality from multiple-cursors.el as well as adding a add-cursors-to-previous-line command to match the add-cursors-to-next-line command.

example.mp4.webm

things I have not done yet:

• fix mark-previous-like-this not working at all
• fix mark-previous-like-this skipping every second match when cursor is to the left of the mark (buffer-point < buffer-mark)
• fix mark-next-like-this always placing fake cursor on the right side of the mark, even when the real cursor is on the left side
  • find a way to start of the mark as the first point marked instead of the top-left always being the start of the mark.
• allow multiple cursors to be on the same line (now cannot be on the same char)
• add tests
• add default keyboard shortcuts
• add description to mark-previous-like-this and mark-next-like-this
• make mark-next/previous-like-this run add-cursors-to-next/previous-line when nothing is marked (to match multiple-cursors.el)
• allow multiple fake cursors to be on the same line, but not on the same point
• fix mark-next-like-this not working with multiple lines
  • initial support
  • fix only deleting the last line marked by the fake cursor (probably something to do with not setting point-line)
  • fix not working when mark does not start at the beginning of the line
• fix multiple cursors being in the same space until something is typed, duplicating the letter typed

issues with multiple cursors out of scope of this PR:

• once you mark something with a fake cursor it has a phantom mark that stays until the fake cursor is removed or until something else is marked.
• undoing an edit done by a fake cursor causes the real cursor to jump to that point.
• in multiple-cursors.el, the fake cursors stay when using keyboard-escape-quit and are only cleared with keyboard-quit. this allows calling keyboard-escape-quit to unmark the current buffer without removing all the fake cursors. in lem there is only keyboard-quit which both unmarks the buffer and removes all the cursors.

I am not very good with lisp so please tell me if there are any problems with my code.
I have moved multiple-cursors.lisp to extensions/ because add-cursors-to-next-like-this uses isearch and I could not figure out how to import the needed functions and commands when it was in lem-core.