Basic type-checking and annotations

[katrinafyi]

It would be nice to have some type-checking (like mypy) which runs on the CI build. This would help catch bugs such as those in #54 and is easier than comprehensive unit testing.

I have vague memories of UQComputingSociety/uqcsbot-slack having this, though I might be mixing it up with ordinary unit tests (which are also not on Discord, but that's #2).

I would do this myself but I suspect the bot is currently in a state of discord.py limbo. Maybe whenever that is sorted out.

[JamesDearlove]

Hoping to have discord.py limbo sorted soon™ @kumar-ish mentioned this in his PR and it's a good idea to have something like this