Add require-nullable-result-in-root lint rule

[nishtahir]

Description

This adds a no-non-null-in-root lint to enforce partial responses which require nullable root elements.

Fixes #1656

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

Screenshots/Sandbox (if appropriate/relevant):

N/A

How Has This Been Tested?

• Unit test - packages/plugin/tests/no-non-null-in-root.spec.ts

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

N/A

[changeset-bot]

🦋 Changeset detected

Latest commit: c750f0c

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

This PR includes changesets to release 1 package

Name	Type
@graphql-eslint/eslint-plugin	Minor

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