Add useValidationRule() plugin

[benjie]

Description

As discussed with @n1ru4l, this adds the useValidationRule() plugin, which can be used to add a validation rule.

Type of change

Please delete options that are not relevant.

• 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

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can
reproduce. Please also list any relevant details for your test configuration

• Added to your test suite

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 on 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 pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

The newrelic tests are failing locally; I think this is unrelated given how trivial the function I've added is.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1f56053

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

This PR includes changesets to release 1 package

Name	Type
@envelop/core	Patch

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[benjie]

I do not know how to address these test failures. I do not think it's related to my changes?