Change webpack peer dependency to optional in loader

Closes GH-2440.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

packages/loader/package.json

@@ -49,6 +49,11 @@
   "peerDependencies": {
     "webpack": ">=5"
   },
+  "peerDependenciesMeta": {
+    "webpack": {
+      "optional": true
+    }
+  },
   "devDependencies": {},
   "scripts": {
     "test": "npm run test-coverage",

packages/loader/readme.md

@@ -35,7 +35,7 @@ This package is a webpack loader to support MDX.
 ## When should I use this?
 
 This integration is useful if you’re using webpack (or another tool that uses
-webpack, such as Next.js).
+webpack, such as Next.js or Rspack).
 
 This integration can be combined with the Babel loader to compile modern
 JavaScript features to ones your users support.