Adds multi-column layout

[vwallen]

Following grid and flexbox, groups the setup properties (column-count, column-width) with the rendering properties (column-gap, column-rule) together into a single feature called "Multi-column Layout" which aligns with how these features show up in posts/documentation. The align-content property follows previous features that include it with the context in which it applies.

Splits out column-fill and column-span into their own features because they apply to the content rather than the layout structure of the columns and/or they are properties of the child elements rather than the parent with the columns declaration.

Adds multi-column group

[ddbeck]

@vwallen If you merge main into the branch and refresh the dist here, this will merge cleanly. Thank you!