DX issue: Using a prefix in v4 requires two actions for a completion

[thecrypticace]

The quickest way for IntelliSense to support the "variant-style" prefix was to treat it as a fake variant and make some tweaks to how and what completions we show. This has an unfortunate side effect of requiring a "double action" whenever typing new classes. We should instead, when no class has been typed yet:

• Offer the prefix as a fake variant like we do today
• Show variants like normal but prefix them with the fake variant so they'll look like tw:md: in the suggestions
• Show classes like normal but prefix them with the fake variant so they'll look like tw:underline in the suggestions

None of this need apply if the variant is already typed nor if the user has typed a class name that does not begin with the variant-style prefix.