[rc/{sway,X}] add custom keyboard layout, mapping Menu to Alt_R

back when i wasn't on wayland, i used to use xmodmap to remap the menu
key to right alt, xmodmap -e "keycode 135 = Alt_R", since, as it turns
out, setxkbmap doesn't have an option for that.

there being no xkb option for changing menu into right alt meant i was
stuck with it on sway, since setxkbmap is X-specific.

figuring out how to write my own xkb rules was "fun".  at first i did
a separate aminb file containing aminb:menu_ralt (à la altwin:menu);
but decided it would be simpler to just define my own us(basic)-based
layout and make all the changes there.

also, no need to map right control to right alt using ctrl:rctrl_ralt
anymore.