Built-in Mud Client features

[KasparMetsa]

We now have

• MMapper WASM build. Player cannot switch mud client by design in WASM build, so the player has to use the built-in mud client. So making the built-in mud client feature-wise similar to the best mud clients is worthwhile because players have a better experience using the WASM build.
• _hotkey feature that allows to configure key bindings in WASM build (and MacOS, Windows, Linux).

What about improving the built-in mud client further? Should we support features such as these in our MMapper built-in client. Some ideas from JMC:
Aliases: ⁨#alias {target} {#var target %1;#showme {>>>Target is now set to "%1" <<<}}⁩
Actions: ⁨#action {^Reconnecting.} {cha wimpy 150}⁩
Variables: ⁨#variable {target} {bear}⁩
Substitutions: ⁨#substitute {%0 'bfzahp ay bfugtizgg'} {%0 'breath of briskness'}⁩
Highlights: ⁨#highlight {red} {an embellished longbow}⁩