feat: Add Directive and DirectiveType

[nzakas]

Prerequisites checklist

• 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?

[mdjermanovic]

We probably also want to re-export Directive here:

rewrite/packages/plugin-kit/src/index.js

Lines 7 to 11 in c95ba1f

	export {
	CallMethodStep,
	VisitNodeStep,
	TextSourceCodeBase,
	} from "./source-code.js";

[nzakas]

Oops, correct!

[mdjermanovic]

LGTM, thanks!