Add next/prev_element_end motions

[leafgarland]

These motions move to the end of the prev/next element. Similar to sexp_move_to_prev_element_tail and sexp_move_to_next_element_tail.

I find them useful for operating on pairs of elements.

I used default mappings of [E and ]E which feel similar to your own defaults.

Thanks for making this project, and for making it really easy to add new motions like these.

[PaterJason]

Thanks for the pr. Those mapping are good for me. Way more vim like than the meta key ones in the original vim-sexp