Fragment Arguments: Spec Implementation

[mjmahone]

This fully specifies the Fragment Arguments changes required, as evidenced by the implementation changes made to graphql-js in https://github.com/graphql/graphql-js/pull/3835

This is an update to a years-old RFC PR https://github.com/graphql/graphql-spec/pull/865, but given that PR had a lot of discussion that has been resolved, it should be easier for reviewers to start with this PR as a cleaner slate. I've tried to incorporate all discussion from previous PRs and Working Group discussion into the initial commit here.

Please use this PR for reviewing and verifying the accuracy of the PR, with inline comments when possible.

For discussions about Fragment Arguments as an idea, missing features or open questions, please open or respond to a thread in the graphql-wg discussion space: https://github.com/graphql/graphql-wg/discussions/1239