Update CSS for type 3 lookups

[RoelN]

Current CSS outputs CSS to only turn on a feature, not to select specific lookups.

E.g. it will output font-feature-settings: "aalt" 1;, turning on the first set of alternates. But if there are multiple sets, the CSS should list them.

Something like:

.bungee-aalt-set1 {
  font-feature-settings: "aalt" 1;
}

.bungee-aalt-set2 {
  font-feature-settings: "aalt" 2;
}