[adapter-vercel] Use path/posix to resolve the relative path (issue#3163) (#3200)

* Use path/posix to resolve the relative path

if we use the default path implementation, on windows we get malformed output since esmodules use "/" as the seperator

(related #3163 )

* use posix as export instead of subpackage

* Add changeset

* Update .changeset/fuzzy-jobs-retire.md

Co-authored-by: Ignatius Bagus <ignatius.mbs@gmail.com>

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Ignatius Bagus <ignatius.mbs@gmail.com>

Author: 3 people

.changeset/fuzzy-jobs-retire.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+Use path.posix to resolve routes for esmodules

packages/adapter-vercel/index.js

@@ -1,5 +1,5 @@
 import { writeFileSync } from 'fs';
-import { relative } from 'path';
+import { posix } from 'path';
 import { fileURLToPath } from 'url';
 import esbuild from 'esbuild';
 
@@ -31,7 +31,7 @@ export default function () {
 
 			builder.log.minor('Generating serverless function...');
 
-			const relativePath = relative(tmp, builder.getServerDirectory());
+			const relativePath = posix.relative(tmp, builder.getServerDirectory());
 
 			builder.copy(files, tmp, {
 				replace: {