Vercel externals (#3614)

* expose external option on adapter-vercel

* update docs

* add an Options type

.changeset/eight-birds-run.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+Expose external option

packages/adapter-vercel/README.md

@@ -14,11 +14,17 @@ import vercel from '@sveltejs/adapter-vercel';
 export default {
 	kit: {
 		...
-		adapter: vercel()
+		adapter: vercel(options)
 	}
 };
 ```
 
+## Options
+
+You can pass an `options` argument, if necessary, with the following:
+
+- `external` — an array of dependencies that [esbuild](https://esbuild.github.io/api/#external) should treat as external
+
 ## Changelog
 
 [The Changelog for this package is available on GitHub](https://github.com/sveltejs/kit/blob/master/packages/adapter-vercel/CHANGELOG.md).

packages/adapter-vercel/index.d.ts

@@ -1,4 +1,8 @@
 import { Adapter } from '@sveltejs/kit';
 
-declare function plugin(): Adapter;
+type Options = {
+	external?: string[];
+};
+
+declare function plugin(options?: Options): Adapter;
 export = plugin;

packages/adapter-vercel/index.js

@@ -6,7 +6,7 @@ import esbuild from 'esbuild';
 const dir = '.vercel_build_output';
 
 /** @type {import('.')} **/
-export default function () {
+export default function ({ external = [] } = {}) {
 	return {
 		name: '@sveltejs/adapter-vercel',
 
@@ -52,7 +52,8 @@ export default function () {
 				outfile: `${dirs.lambda}/index.js`,
 				target: 'node14',
 				bundle: true,
-				platform: 'node'
+				platform: 'node',
+				external
 			});
 
 			writeFileSync(`${dirs.lambda}/package.json`, JSON.stringify({ type: 'commonjs' }));