fix(core): Implement stricter variable typings on generic

[kitten]

Resolve #2606

Summary

This attempts to introduce a stricter type for variables preventing it from being an optional property or argument (i.e. variables?: Variables) in all parts of the codebase. To do this, we also need to make its type a little more strict with AnyVariables, which explicitly defaults the generic to a type of void | undefined | { ... }. In case of a generic that's passed in, this also becomes the extension type.

This is currently a draft because some methods on Client and some utility functions, like createRequest, are affected by this change. We have to check that these behave as expected (as per the change and RFC) and that we don't break them if that isn't necessary

Set of changes

• Add AnyVariables and make generics Variables extends AnyVariables = AnyVariables
• Update core typings where variables were optional
• Update all typings across bindings (wherever variables?: was used, a special type must be implemented)

[changeset-bot]

🦋 Changeset detected

Latest commit: 9aca906

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@urql/exchange-graphcache	Major
@urql/core	Major
@urql/preact	Major
urql	Major
@urql/svelte	Major
@urql/vue	Major
next-urql	Major

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[JoviDeCroock]

These come from #2608

[crmitchelmore]

@kitten is the expected pattern to pass empty variables for cases when there are no variables?

e.g. client.query(SomeDocument)

becomes:

client.query(SomeDocument, {})