Change Request: Export types from main entry point

### Environment

ESLint version: HEAD
@eslint/json version: HEAD
Node version: 20.19.3
npm version: 10.9.2
Operating System: Windows 11

### What problem do you want to solve?

Hi,

I'm raising this issue based on https://github.com/eslint/markdown/pull/453, https://github.com/eslint/markdown/pull/367#issuecomment-3008282520, and https://github.com/eslint/markdown/pull/367#issuecomment-3028229001.

cc. @JoshuaKGoldberg

---

Currently, the JSON, CSS, and Markdown plugins export their types defined in `types.ts` from the `/types` entry point.

For example:

- JSON: `@eslint/json/types`
- [package.json reference](https://github.com/eslint/json/blob/main/package.json#L20-L27)
- [`types.ts` reference](https://github.com/eslint/json/blob/main/src/types.ts)

https://github.com/eslint/json/blob/bc2b31768b0e7dcde61d456ab34bafa3afce65a8/package.json#L20-L27

- CSS: `@eslint/css/types`
- [package.json reference](https://github.com/eslint/css/blob/main/package.json#L20-L27)
- [`types.ts` reference](https://github.com/eslint/css/blob/main/src/types.ts)

- Markdown: `@eslint/markdown/types`
- [package.json reference](https://github.com/eslint/markdown/blob/main/package.json#L18-L20)
- [`types.ts` reference](https://github.com/eslint/markdown/blob/main/src/types.ts)

---

As mentioned in https://github.com/eslint/markdown/pull/453, https://github.com/eslint/markdown/pull/367#issuecomment-3008282520, and also from my own experience, exporting types from a `/types` entry point is a fairly uncommon pattern in TypeScript.

With this proposal, I'd like to suggest exporting types from the main entry point instead (i.e., `@eslint/json`, `@eslint/css`, and `@eslint/markdown`), rather than from `@eslint/json/types`, `@eslint/css/types`, and `@eslint/markdown/types`.

### What do you think is the correct solution?

There are two possible approaches, depending on whether the build script uses bundling:

- If bundling is used:

Adding `export * from "types.ts"` to the `banner` section in `rollup.config.js` worked well and was the simplest way to achieve this proposal in my tests.

https://github.com/eslint/json/blob/bc2b31768b0e7dcde61d456ab34bafa3afce65a8/rollup.config.js#L14

- If bundling is not used:

The following approach was the cleanest solution I found.

> I've taken another shot at this and wanted to share a different approach.
>
> Here's what I did:
>
> - First, I created an empty `types.js` file at `src/types.js`:
>
> ```js
> export default {};
> ```
>
> - Then, I added `export * from "./types.js";` to the end of `index.js`:
>
> ```diff
> // ...
>
> export default plugin;
> export { MarkdownSourceCode };
> + export * from "./types.js";
> ```
>
> ---
>
> This approach seems to work well, since it allows all types from `src/types.ts` to be imported directly from the `@eslint/markdown` package, rather than from `@eslint/markdown/types`.
>
> 
>
> 

### Participation

- [x] I am willing to submit a pull request for this change.

### Additional comments

This proposal applies to all packages that export their types from `types.ts` using the `@eslint/package-name/types` entry point.

So far, I've only reviewed the JSON, CSS, and Markdown plugins, but I plan to look into the other packages as I continue.