Use changesets for Primer CSS

[simurai]

Problem

Currently we use the #1131 PR to update dotcom. This works great for the many color modes updates, but we currently don't have a good process of making other changes. We could just merge everything into the mkt/color-modes-whee branch, but that makes changes hard to track down.

Possible solution

We started to use changesets for other Primer repos. So we might could try it for Primer CSS too? Especially because the color modes work will go on for while and it's tedious to make new releases manually.

• Docs (internal)
• Example PR Set up changesets react#967 for Primer React Components.