add esbuildOptions param to getConfig

[Vahor]

Currently it only accepts "external" prop as other fields might break the config build process

fix: #70

To test this, I updated the `next-contentlayer-example` locally with

In file next.config.js

const {
	createContentlayerPlugin,
	defaultPluginOptions,
} = require("next-contentlayer2");

/** @type {import('next').NextConfig} */
const nextConfig = {};

const withContentlayer = createContentlayerPlugin({
	...defaultPluginOptions,
	esbuildOptions: {
		external: ["@terrastruct/d2"],
	},
});
module.exports = withContentlayer(nextConfig);

package.json

{
	"name": "next-contentlayer-example",
	"version": "0.1.0",
	"private": true,
	"scripts": {
		"dev": "next dev",
		"build": "next build",
		"start": "next start"
	},
	"dependencies": {
+		"@terrastruct/d2": "^0.1.23",
+		"contentlayer2": "workspace:*",
		"date-fns": "4.1.0",
		"next": "^15.2.4",
+		"next-contentlayer2": "workspace:*",
		"react": "^19.1.0",
		"react-dom": "^19.1.0"
	},
	"devDependencies": {
		"@tailwindcss/postcss": "4.0.17",
		"@types/react": "19.0.12",
		"postcss": "^8.4.24",
		"tailwindcss": "4.0.17",
		"typescript": "^5.5.0"
	}
}

contentlayer.config.js

+ import { D2 } from "@terrastruct/d2";


+const superRehypePlugin = () => {
+  const d2 = new D2();
+
+  return async (tree: unknown) => {
+    const render = await d2.compile("a -> b");
+    return tree;
+  };
+};

export default makeSource({
  contentDirPath: 'posts',
  documentTypes: [Post],
+  mdx: {
+   rehypePlugins: [superRehypePlugin],
+  },
})

If you uncomment external: ["@terrastruct/d2"], or remove that section entirely, the build / dev will fail.

I haven't found any tests that I could update for this usecase, If you want to add one tell me.

I also did not test the cli, only ran the main command to check if the external param was there

[timlrx]

Thanks much appreciated. I think the approach to just expose the external field for esbuildOptions is good for now.