replace @babel packages that have moved from "proposed" to "official"

Summary: Use official plugin names for syntax extensions that moved from "proposed" to "official", e.g. plugin-proposal-private-methods is now plugin-tranform-private-methods. 

Details: we reference several libraries in webpack/babel-options.js that previously were provided by @babel/preset-env. Technically, we should always have included them in our package.json as direct dependencies, but since @babel/preset-env also depended on them we never noticed they were missing. Recently, however, babel replaced many of instances of @babel/plugin-proposal-foo with @babel/plugin-transform-foo, causing STRWEB-86. We added these missing dependencies to correct this omission, but the Even More Correct fix is to update both babel-options.js and package.json to use the new, official transforms instead of the old, proposed transforms.