Add devDependency and loader util for additional bundler

This adds `@rspack/core` as a `devDependency` to Next.js, as it will be conditionally referenced in core in the future and will only run when the real dependency is added via a future plugin.

This also creates a `getRspackCore` utility which performs this check, suppresses the lint, and more clearly informs the user of the error.

package.json

@@ -102,6 +102,7 @@
     "@next/third-parties": "workspace:*",
     "@opentelemetry/api": "1.4.1",
     "@picocss/pico": "1.5.10",
+    "@rspack/core": "npm:@rspack-canary/[email protected]",
     "@svgr/webpack": "5.5.0",
     "@swc/cli": "0.1.55",
     "@swc/core": "1.9.3",

packages/next/src/shared/lib/get-rspack.ts

@@ -0,0 +1,16 @@
+import type * as RspackCore from '@rspack/core'
+
+export function getRspackCore(): typeof RspackCore {
+  try {
+    // eslint-disable-next-line import/no-extraneous-dependencies
+    return require('@rspack/core')
+  } catch (e) {
+    if (e instanceof Error && 'code' in e && e.code === 'MODULE_NOT_FOUND') {
+      throw new Error(
+        '@rspack/core is not available. Please make sure the appropriate Next.js plugin is installed.'
+      )
+    }
+
+    throw e
+  }
+}