Add first draft of Popup (Editor's Draft) (#355)

* [Popup spec] Add brief intro with HTML-esque metadata

* [Popup] Fix up frontmatter

* [Popup] Add spec outline

* [Popup] Add initiallyopen attribute (addresses #311)

* [Popup] Add show() method

* [Popup] Convert a note to a todo

* [Popup] Add popup attribute

* [Popup] Hide currently-shown popup elements steps

* [Popup] Setting focus when the popup element is shown

* [Popup] Hiding a popup element

* [Popup] Componentize 'showing a popup element steps'

* [Popup] Various and sundry edits

* [Popup] Address first ~half of first round of feedback

* [Popup] 2021.07.01 checkpoint in addressing PR feedback (also addresses #358)

* [Popup] 2021.07.02 spec updates

* [Popup] First batch of updates to HTML editor feedback

* Apply suggestions from HTML editor code review

Co-authored-by: Domenic Denicola <[email protected]>

* 2021.08.24 batch of popup updates for HTML editor feedback

Co-authored-by: Domenic Denicola <[email protected]>