Checkpoint for experimental incremental delivery support

[glasser]

It basically works, modulo a bunch of concerns:

• requires unreleased graphql-js (and installing with npm i --legacy-peer-deps)
• that graphql-js doesn't work with CJS (and currently doesn't work with our test suite for related reasons)
• graphql-js could have some typing improvements (eg FormattedExecutionResult isn't fully formatted)
• in general, the impact on the plugin API needs thought
• response cache needs updating so it doesn't cache things too early
• docs need updating

Fixes #6671.

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