Add type definitions for next/mdx (#36815)

Although it’s not intended for use in TypeScript, TypeScript still offers some nice features when it’s used in next.config.js (completion, hovers, `"checkJs": true`).

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

package.json

@@ -49,6 +49,7 @@
  "@babel/core": "7.15.0",
  "@babel/eslint-parser": "7.15.0",
  "@babel/parser": "7.12.11",
+ "@babel/generator": "7.15.0",
  "@babel/plugin-proposal-object-rest-spread": "7.14.7",
  "@babel/preset-flow": "7.14.5",
  "@babel/preset-react": "7.14.5",

packages/next-mdx/index.d.ts

@@ -0,0 +1,33 @@
+/* eslint-disable */
+import { NextConfig } from 'next'
+import { CompileOptions } from '@mdx-js/mdx'
+import { RuleSetConditionAbsolute } from 'webpack'
+
+type WithMDX = (config: NextConfig) => NextConfig
+
+interface NextMDXOptions {
+ /**
+ * A webpack rule test to match files to treat as MDX.
+ *
+ * @default /\.mdx$/
+ * @example
+ * // Support both .md and .mdx files.
+ * /\.mdx?$/
+ */
+ extension?: RuleSetConditionAbsolute
+
+ /**
+ * The options to pass to MDX.
+ *
+ * @see https://mdxjs.com/packages/mdx/#api
+ */
+ options?: CompileOptions
+}
+
+/**
+ * Use [MDX](https://github.com/mdx-js/mdx) with [Next.js](https://github.com/vercel/next.js)
+ */
+declare function nextMDX(options?: NextMDXOptions): WithMDX
+
+export = nextMDX
+/* eslint=enable */