feat: adapter-vercel should generate sourcemaps (#5197)

* feat: enable sourcemap support

The adapter-vercel package were not generating sourcemaps. Therefore, it was impossible to correctly use any bug tracking tool.

* fix: add missing changeset

* style: fix README.md indentation

* fix: always generate linked sourcemaps and delete configuration option

.changeset/mighty-melons-walk.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+Add sourcemap generation for the generated functions

packages/adapter-vercel/index.js

@@ -146,7 +146,8 @@ async function v1(builder, external) {
 		bundle: true,
 		platform: 'node',
 		external,
-		format: 'cjs'
+		format: 'cjs',
+		sourcemap: 'linked'
 	});
 
 	fs.writeFileSync(`${dirs.lambda}/package.json`, JSON.stringify({ type: 'commonjs' }));
@@ -302,7 +303,8 @@ async function v3(builder, external, edge, split) {
 			bundle: true,
 			platform: 'node',
 			format: 'esm',
-			external
+			external,
+			sourcemap: 'linked'
 		});
 
 		write(