feat(deps): bundle postcss (#6)

We move `postcss` from `dependencies` to `devDependencies` to allow
Rslib to bundle the it into `dist/`.

Author: colinaaa

package.json

@@ -27,16 +27,14 @@
   "simple-git-hooks": {
     "pre-commit": "npm run lint:write"
   },
-  "dependencies": {
-    "postcss": "^8.4.47"
-  },
   "devDependencies": {
     "@biomejs/biome": "^1.9.4",
     "@playwright/test": "^1.48.2",
     "@rsbuild/core": "^1.1.0",
     "@rslib/core": "^0.0.16",
     "@types/node": "^22.9.0",
     "playwright": "^1.48.2",
+    "postcss": "^8.4.47",
     "simple-git-hooks": "^2.11.1",
     "tailwindcss": "^3.4.14",
     "typescript": "^5.6.3"