fix: loader support for server-only (#6383)

## Description

Allows `server-only` to work within the Payload loader.

Fixes https://github.com/payloadcms/payload-3.0-demo/issues/218

test/loader/int.spec.ts

@@ -11,6 +11,11 @@ describe('Loader', () => {
     expect(code).toStrictEqual(0)
   })
 
+  it('should import server-only without breaking', async () => {
+    const code = await startChildProcess('./server-only-test.ts')
+    expect(code).toStrictEqual(0)
+  })
+
   it('should import configs that rely on custom components', async () => {
     const code = await startChildProcess('../admin/config.ts')
     expect(code).toStrictEqual(0)

test/loader/server-only-test.ts

@@ -0,0 +1,3 @@
+import 'server-only'
+
+console.log('woo')

test/package.json

@@ -48,6 +48,7 @@
     "execa": "5.1.1",
     "http-status": "1.6.2",
     "payload": "workspace:*",
+    "server-only": "^0.0.1",
     "tempy": "^1.0.1",
     "ts-essentials": "7.0.3",
     "typescript": "5.4.5",