feat: support legacy `@core` link syntax

[goto-bus-stop]

This is for backward compatibility with existing supergraphs, bolting @core support onto the @link reading. It does not do anything to support fed v1 supergraphs.

While @core is not used in new graphs today, it is supported by the JS code, and we have a lot of tests throughout our systems that rely on using @core with fed v2.

Additionally, because the implementation of API schema generation is the same between fed v1 and fed v2, supporting @core will let us land it in the router without a fallback to JS for fed v1 supergraphs.

All new code that supports @core mentions @core literally in a comment so we can rip it out easily when we migrate over customers and our tests in the router.

[goto-bus-stop]

Marking the comments resolved because we'll follow the JS code rather than the spec (or good sense 😄 ). this just needs a rubber-stamp approval I'd say