Use package.json exports when ts support lands in

[eric-burel]

Is your feature request related to a problem? Please describe.
@see https://stackoverflow.com/questions/63058081/package-json-with-multiple-entrypoints
@see https://blog.mozilla.org/data/2021/04/07/this-week-in-glean-publishing-glean-js/

This would allow us to use index.server.ts files instead of server/index.ts, and get rid of the typesVersions hack we currently use to handle both server and client exports.

Also see this Lerna ticket for the "fake alias" hack: @see lerna/lerna#3006