Better abstract string concatenation

Due to the way we construct type conversions, oftentimes we see unidiomatic code like https://github.com/jsdom/webidl2js/blob/7d45dfc6e6d1ba737b77eaabbab712dcf2d19435/test/__snapshots__/test.js.snap#L167 Ideally we want to merge string literals together and even use template literals.

Related to this is #204, but this seems mostly orthogonal and more tractable.