feat: add CustomRuleVisitorWithExit type to @eslint/plugin-kit

[Pixel998]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR adds a CustomRuleVisitorWithExit type to the @eslint/plugin-kit package to address the need for a shared type that can generate exit selectors for rule visitors. Currently, each language plugin (JavaScript, JSON, CSS, Markdown) has to reimplement this same helper type separately.

What changes did you make? (Give an overview)

• Added a new CustomRuleVisitorWithExit type.
• Added type tests for CustomRuleVisitorWithExit

Related Issues

Fixes #318

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

[fasttime]

We also need to export CustomRuleVisitorWithExit from packages/plugin-it/source-code.js, probably like this:

/** @typedef {import("@eslint/core").RuleVisitor} RuleVisitor */
/**
 * @typedef {import("./types.ts").CustomRuleVisitorWithExit<RuleVisitorType>} CustomRuleVisitorWithExit<RuleVisitorType>
 * @template {RuleVisitor} RuleVisitorType
 */

The next step is ensuring that packages/plugin-kit/dist/cjs/index.d.cts references import("./types.cts") instead of import("./types.ts"). This kind of substitution is already being done in tools/build-cts.js, but only for static imports:

rewrite/tools/build-cts.js

Lines 27 to 30 in f8df139

	const newSourceText = oldSourceText.replaceAll(
	' from "./types.ts";\n',
	' from "./types.cts";\n',
	);

[fasttime]

LGTM, thanks! Leaving open for a second review.