`embed` function handle usage for Observable

[etowahadams]

Make the "core" embed function handle usage in Observable. Basically overload the "core" embed to do what embed does in embed/index.ts, creating an element depending on the number of arguments and returning the wrapper. We have this code as vega-embed as reference.

from: https://github.com/gosling-lang/gosling.js/pull/977#discussion_r1359665192