feat: add @rspack/core as an optional peer dependency (#679)

Author: snitin315

package.json

@@ -41,9 +41,18 @@
     "dist"
   ],
   "peerDependencies": {
+    "@rspack/core": "0.x || 1.x",
     "postcss": "^7.0.0 || ^8.0.1",
     "webpack": "^5.0.0"
   },
+  "peerDependenciesMeta": {
+    "@rspack/core": {
+      "optional": true
+    },
+    "webpack": {
+      "optional": true
+    }
+  },
   "dependencies": {
     "cosmiconfig": "^9.0.0",
     "jiti": "^1.20.0",