I wanted to give you a jump start on updating Custom CSS in Jetpack with the new code editor APIs in 4.9-alpha. This is just an initial integration which fixes the JS error that was caused by Jetpack and core conflicting. I'm sure there is additional integration work needed, specifically in the realm of switching between CSS, LESS, and SCSS. But I hope this is helpful for you to get started and also to evaluate the proposed core changes in Trac #41897 which makes the Custom CSS control in the Customizer actually extensible (please apply that core patch or checkout the branch to test). Any findings from the Jetpack integration here will inform what changes need to be made in that core patch.

So I hand this over to the Jetpack team to amend as needed.

Depends on xwp/wordpress-develop#257 which resolves https://core.trac.wordpress.org/ticket/41897

Fixes #7776.

The dependency xwp/wordpress-develop#257 has been merged into trunk as r41558, and so wpcore#41897 is now resolved and there are no other dependencies for testing this PR, other than to be on trunk.

Closing in favour of #8028. (The commit here is included in the other PR)