[FIX] Use the 'browser' platform setting for esbuild for Vercel Edge Functions (#6109)

* Use browser platform setting for esbuild

This should use the same esbuild settings as the Cloudflare adapter, as Vercel Edge Functions are based on Cloudflare Workers.

* changeset

Author: SG60

.changeset/wet-crews-smoke.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+Use `platform: 'browser'` for esbuild in Vercel Edge Functions.

packages/adapter-vercel/index.js

@@ -186,7 +186,7 @@ export default function ({ external = [], edge, split } = {}) {
 					outfile: `${dirs.functions}/${name}.func/index.js`,
 					target: 'es2020', // TODO verify what the edge runtime supports
 					bundle: true,
-					platform: 'node',
+					platform: 'browser',
 					format: 'esm',
 					external,
 					sourcemap: 'linked'