Add Rspack optional peer dependency for @tailwindcss/webpack (#20027)

## Summary

This PR makes Rspack support explicit for `@tailwindcss/webpack` by
adding `@rspack/core` as an optional peer dependency alongside
`webpack`.

The loader already works through Rspack's webpack-compatible loader API,
as shown in this runnable example:
https://github.com/rstackjs/rstack-examples/tree/main/rspack/tailwindcss.
The README and package description are updated to document that usage.

## Test plan

No Rspack-specific tests were added. The loader implementation is
unchanged, and the existing webpack loader tests exercise the same
loader API path that Rspack uses, which should cover the relevant
behavior.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

packages/@tailwindcss-webpack/package.json

@@ -41,6 +41,15 @@
     "webpack": "catalog:"
   },
   "peerDependencies": {
-    "webpack": "^5"
+    "@rspack/core": "^1.0.0 || ^2.0.0",
+    "webpack": "^5.0.0"
+  },
+  "peerDependenciesMeta": {
+    "@rspack/core": {
+      "optional": true
+    },
+    "webpack": {
+      "optional": true
+    }
   }
 }