[popup] Allow <div popup> as a shorthand for <div popup=popup>

[zcorpan]

https://open-ui.org/components/popup.research.explainer#html-content-attribute

A new content attribute, popup, controls both the top layer status and the dismiss behavior. There are several allowed values for this attribute:

• popup=popup - A top layer element following “Popup” dismiss behaviors (see below).
• popup=hint - A top layer element following “Hint” dismiss behaviors (see below).
• popup=async - A top layer element following “Async” dismiss behaviors (see below).

The proposal for popups is now a global attribute instead of an element or a set of elements for popups, as discussed in #410 (comment)

For developer ergonomics, I suggest allowing the empty string as a valid value that is a synonym to popup. That is, these should be equivalent:

<div popup=popup></div>

<div popup></div>

Since popup=popup looks the same as a boolean attribute, developers may expect the short form popup to work.

There are some enumerated attributes in HTML where the empty string is a valid keyword: crossorigin, hidden, contenteditable, translate, preload, spellcheck.

cc @mfreed7

[gregwhitworth]

Closing as the boolean option is being discussed in 491