refactor: Flesh out 'Todo List' REPL example to match 'Todo List (Signals)'

[rschristian]

Someone mentioned this ages ago and it's been on my todo list (hehe) since.

If we have Todo List and Todo List (Signals), it should be expected that the functionality otherwise matches and is more useful at comparing the class component to signals implementations. I tried to keep them looking pretty similar where possible, even if this might be a bit more verbose than you'd usually see.

Supersedes #822