Untangling emotion from source-foundations

[ashishpuliyel]

Background

In some cases, we are exporting Emotion CSS from source-foundations. This goes against the principle that source-foundations should be agnostic as to which css-in-js frameworks it supports.

It also breaks the build for AMP in DCR, because it makes the assumption that some of the CSS we export is not Emotion CSS.

Work done

Original PR that was closed and split into the two below for clarity:
#1554

• Fixes CSS and adds tests to ensure it is valid: Check that we are exporting valid CSS #1566
• Proposal for a possible approach to indicate where we export Emotion CSS: Clearly distinguish between CSS and Emotion CSS #1568

[ob6160]

Following some discussion on #1568, we've decided to pause work on this and discuss our approach towards removing the dependency on emotion that we have in source-foundations instead of potentially setting it in stone by adding it as a peer dependency.