feat: Add Directive and DirectiveType

[nzakas]

Prerequisites checklist

• [x] I have read the contributing guidelines.

What is the purpose of this pull request?

Add Directive class to @eslint/plugin-kit. (Extracted from the core.)

What changes did you make? (Give an overview)

• Added a Directive class to @eslint/plugin-kit to be used when implementing SourceCode#getDisableDirectives()
• Added a DirectiveType type to @eslint/core
• Changed Directive#node to be type unknown instead of object to force consumers to type cast the value before using it.
• Added tests for Directive

Related Issues

Is there anything you'd like reviewers to focus on?

[nzakas]

Oops, correct!