fix: add support for exactOptionalPropertyTypes in type definitions

[sebastian-altamirano]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

When using @eslint/markdown as a plugin in an ESLint config file, TypeScript throws the following error if the exactOptionalPropertyTypes compiler option is enabled:

Type 'typeof plugin' is not assignable to type 'Plugin'.
...
    Type 'undefined' is not assignable to type '(string | string[])[]'.ts(2322)

For example, this can be triggered by using plugins: { markdown } in:

import markdown from "@eslint/markdown";
import { defineConfig } from "eslint/config";

export default defineConfig([
    {
        extends: ["markdown/recommended"],
        files: ["**/*.md"],
        language: "markdown/gfm",
        plugins: { markdown },
        rules: {
            "markdown/no-duplicate-headings": "error",
            "markdown/no-html": "error",
        },
    },
]);

This PR fixes the issue.

What changes did you make? (Give an overview)

I noticed that the Plugin type was unused, so I updated it to be compatible with exactOptionalPropertyTypes and began using it, without breaking existing tests.

Related Issues

-

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

-

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