Consider swapping the M-RET and S-<return> bindings

This would be a change for users (and take some adjusting to), but it looks like `S-` isn't normally available in terminals – so should the standard bindings for `M-RET` (which is available in terminals) and `S-` (which isn't) be swapped?

Being able to reply to messages is, after all, far more important than selecting between two alternatives for writing a new message (and I think even more so now that users can customize their preferred `ement-room-compose-method`).

I noticed this issue when updating the transient menu in https://github.com/phil-s/ement.el/commit/24197ba25099d478e6cc97298e8972554e6c2b2f and realising that the transient specified `S-RET` (vs `S-` in the regular keymap), and then establishing that `S-RET` (which *looks* like it would be terminal-friendly) didn't seem to be a thing in practice.