Stop build warning about experimental: { esmExternals: 'loose' } (#40377)

Fixes #40368.

## Bug

- [x] Related issues linked using `fixes #number`
- [ ] ~~Integration tests added~~ [none applicable]
- [ ] ~~Errors have helpful link attached, see `contributing.md`~~ [no new errors]

packages/next/server/config-schema.ts

@@ -253,7 +253,14 @@ const configSchema = {
           type: 'boolean',
         },
         esmExternals: {
-          type: 'boolean',
+          oneOf: [
+            {
+              type: 'boolean',
+            },
+            {
+              const: 'loose',
+            },
+          ] as any,
         },
         externalDir: {
           type: 'boolean',