v6: More utilities API, docs, tests (#42381)

* Utilities API and class improvements

* Expand utility API test coverage

* Update utilities API and border-radius docs

* Fix bundlewatch

* Introduce `@mixin generate-utilities-loop()`

* Add text wrapping utilities in Sass utilities API in the docs

* Copy updates, links, callout for postcss prefix, fix a border-radius

* inline warning

* error over warn

* Fix bundlewatch

* more copy edits

* fix layout and headings

* lint

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Co-authored-by: Julien Déramond <juderamond@gmail.com>