Add join__FieldSet, link__Import, link__Purpose to stripFederationFromSupergraph

[kamilkisiela]

Closes #14

[github-actions[bot]]

🚀 Snapshot Release (alpha)

The latest changes of this PR are available as alpha on npm (based on the declared changesets):	Package	Version	Info
@theguild/federation-composition	0.1.2-alpha-20231006074034-05528f3	npm ↗︎ unpkg ↗︎