Add ability to detect unused variables

[mfn]

Summary

Inspired by webonyx/graphql-php#615 (comment)

I once had a bug in production which went unnoticed for some time because one of the "variables" passed it had a typo and thus wasn't consumed. The destination field in question also was "nullable", so no obvious error was triggered.

The idea is to provide a config option to enable this behaviour (e.g. only on local development) to throw a hard error.

Example

Query

mutation test($value:ID) {
  someMutation(type:"falbala", optional_id:$value)
}

Variables:

{
  // Ops! typo
  "values": "159"
}

Result:

{
  "errors": [
    {
      "message": "The following variables were provided but not consumed: values",
      "extensions": {
        "category": "graphql"
      }
    }
  ]
}

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[mfn]

Finally found time to wrap this up \o/