Generators.{now,width}

[mbostock]

This moves now() and width(target?) to Generators.now() and Generators.width(target, options?) in the vanilla standard library. The goal of this change is:

• Make the difference easier to talk about; are you referring to the built-in width available in Observable Markdown, or the Generators.width available in vanilla JavaScript? If they are both called “width” it’s confusing.
• Make it possible to use Generators.width in Observable Markdown to observe the width of something else; you can even override the built-in width by saying const width = Generators.width(document.body).
• Make the target argument mandatory to Generators.width so it’s unambiguous which element you’re observing.