feat: add cursor option to the shell command

[HE7086]

Add a cursor option to shell command, similar to rename.
This allows user to define keymaps with cursor set to generic place, so that they don't have to manually move the cursor every time. When the option is unspecified the behavior is same as before (at the end).
Some examples:

[manager]
prepend_keymap = [
  # cursor at end, same as before
  { on = [ ":" ], run = 'shell --block --interactive "vim $@"' },
  # cursor at start, user can input command directly, e.g. vim
  { on = [ ":" ], run = 'shell --block --interactive --cursor=start "$@"' },
  # cursor after 7th char (command), user can input flags directly.
  { on = [ ":" ], run = 'shell --block --interactive --cursor=7 "command $@"' },
]

--cursor=start is equivalent to --cursor=0. I added it here because it is supported by rename already. There could be some extra variants like after_first, before_last if necessary. --cursor=<usize> is sufficient for most cases.

[sxyazi]

Thanks!