set not modifiable when possible

[emmanueltouzery]

it's a little jarring that it's possible to edit the contents of the any-jump window.

I tried to add:

  call nvim_buf_set_option(buf, 'modifiable', v:false)

on line 226 in the script and that helps, but i really don't know the script enough to know what it could all break. i definitely doubt that this wouldn't break anything. but something like that, but correct, would be a nice improvement, i think!