[css-gaps-1] Initial editor's draft. w3c#10393 (w3c#11115)

* Copy spec skeleton from kbabbitt.github.io/css-gap-decorations

* Update metadata

* Update anchor definition for masonry containers

* Major editing pass

- Defer out gap decoration areas
- Remove direction-relative properties
- Flesh out definitions and algorithms
- Add examples

* Fold the 'join' behavior from outset properties into break properties

Plus minor editorial changes.

* Move break and outset properties closer to corresponding algorithms

* Clarify and simplify repeat algorithm

* Address textual suggestions

* Add visuals for paint order

* More visuals and formatting

* Minor grammar fix

* Add explanation for spanning-item test

* Minor grammatical fixes

* Fix bikeshed linking errors

* Fix definition of crossing gap width

Author: kbabbitt