Linux

I'm pretty sure both are possible in xkb. But you'll have to learn how to get a custom xkb_keymap into your DE of choice. I only learned enough to do one mapping:

xkb_keymap {
	xkb_keycodes  { include "evdev+aliases(qwerty)" };
	xkb_types     { include "complete" };
	xkb_compat    { include "complete" };
	xkb_symbols   {
		include "pc+us+inet(evdev)"
		key  {
			type= "TWO_LEVEL",
			symbols[Group1] = [  Multi_key,                Caps_Lock ],
			actions[Group1] = [ NoAction(), LockMods(modifiers=Lock) ]
		};
		key  {[ Escape ]};
	};
	xkb_geometry  { include "pc(pc105)" };
};

This remaps Capslock to Escape, Escape to Compose, and Shift+Escape to Capslock. Not what you want, but hopefully this will give you a starting point to playing/breaking xkb.

Another benefit of doing this with xkb: it's now a separate codebase from X.org, and is used in every Wayland compositor I know of.

For shells (and other programs) using GNU readline for interactions and line-edits (like bash), some of this can be achieved with an ~/.inputrc configuration file, e.g., mapping the correct key sequence for your terminal emulator to the backward-word move command. You can look up these sequences using infocmp -L1 or interactively using sed -n l.

Most other shells use their own command line handling routines and configuration though, so this won't work for e.g., zsh or fish.

Now does this have anything to do with why my ZSH outputs ;5D everytime I press Ctrl+Left Arrow?