Add missing `@babel/plugin-*` to dependencies

For future archaeologists, from the conversation on Slack:

do we have any idea why we’re seeing this now? Maybe this is another case of “We relied on X and Y but didn’t realize we didn’t actually depend on Y because X supplied it. Then X changed its internal impl to avoid Y, but of course we still rely on it, and 

 ”
 
[Yep] we used to leverage these libraries via @babel/preset-env. Specifically: several proposals recently became standards and babel merged a PR last week that replaced thousands of instances of @babel/plugin-proposal-foo with @babel/plugin-transform-foo. IDK why that was published in a patch release instead of a breaking change 

 but at least I understand what happened to us.