Improve type for configEmoji option

[bmish]

Use a tuple with named elements instead of just a string array. This provides more information about the expected type, and allows the type-checker (when present) to catch issues sooner instead of depending on runtime checks. This is not a breaking change because we already enforced the same checks at runtime.

As a reminder, the way this option is used is for specifying an emoji for each config like this:

{
  configEmoji: [
    ['recommended', '🔥'],
    ['stylistic', '🎨'],
  ],
}