Expand emoji detection to detect Slack emojis

[mbernson]

Hey Jaap, here's my quadrennial pull request to this project :)

I noticed that when Haikubot gets a haiku wrong, it's often related to the emoji in the message.
Haikubot sees them like Slack emoji codes (:joy: for example). This PR updates the internal emoji detection logic to treat those as if they were regular emoji.

[mbernson]

Currently, emojis are considered to be a single-syllable word. Should we filter out the emoji entirely instead?

function expandMessage(msg) {
  return msg
    .split(/\s+/)
    .filter((x) => x.length > 0 && x != "\u200B")
    .filter((x) => !isEmoji(x));
}