[2024-05] Field a proposal to replace `instanceof`

[kitten]

Write a proposal to replace instanceof checks (implemented via https://github.com/graphql/graphql-js/blob/main/src/jsutils/instanceOf.ts).

These are currently used for internal schema language class instance checks and an error has been put in place to prevent two instances of the graphql package from cooperating.

This also contains a dev/prod split and check that's causing other issues.

We may want to replace this with a warning (rather than an error) to prevent breaking changes, and replace the general approach on how we identify schema language class instances with something else (what this proposal should define)

• assignee(s): @kitten
• source:
• expecting to check in with: @JoviDeCroock @jerelmiller @phryneas

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