introduce NextConfigExperimental as suggested in PR review

Author: dario-piotrowicz

packages/next-on-pages/build-metadata.d.ts

@@ -7,10 +7,8 @@ type NextOnPagesBuildMetadata = {
 	/** (subset of) values obtained from the user's next.config.js (if any was found) */
 	config?: {
 		experimental?: Pick<
-			NonNullable<
-				// eslint-disable-next-line @typescript-eslint/consistent-type-imports -- the import needs to be dynamic since the nextConfig file itself uses this type
-				import('./src/buildApplication/nextConfig').NextConfig['experimental']
-			>,
+			// eslint-disable-next-line @typescript-eslint/consistent-type-imports -- the import needs to be dynamic since the nextConfig file itself uses this type
+			import('./src/buildApplication/nextConfig').NextConfigExperimental,
 			'allowedRevalidateHeaderKeys' | 'fetchCacheKeyPrefix'
 		>;
 	};

packages/next-on-pages/src/buildApplication/nextConfig.ts

@@ -9,11 +9,13 @@ import * as os from 'os';
  * version of it which includes just what we need in next-on-pages.
  */
 export type NextConfig = Record<string, unknown> & {
-	experimental?: {
-		incrementalCacheHandlerPath?: string;
-		allowedRevalidateHeaderKeys?: string[];
-		fetchCacheKeyPrefix?: string;
-	};
+	experimental?: NextConfigExperimental;
+};
+
+export type NextConfigExperimental = {
+	incrementalCacheHandlerPath?: string;
+	allowedRevalidateHeaderKeys?: string[];
+	fetchCacheKeyPrefix?: string;
 };
 
 /**