Add Prettier to dev dependencies

I'm trying out a new editor setup, and it noticed that Prettier isn't
obviously installed! I think it makes sense to put it in dev deps, even
if there's not a direct hook calling it—though tbh maybe I should add it
to `yarn dev` somehow?

package.json

@@ -26,6 +26,9 @@
     "react-transition-group": "^4.3.0",
     "tweenjs": "^1.0.2"
   },
+  "devDependencies": {
+    "prettier": "^3.0.3"
+  },
   "scripts": {
     "build": "esbuild app/javascript/*.* --bundle --sourcemap --outdir=app/assets/builds --public-path=/assets --asset-names=[name]-[hash].digested --loader:.js=jsx --loader:.png=file --loader:.svg=file --loader:.min.js=text",
     "dev": "yarn build --watch --public-path=/dev-assets"

yarn.lock

@@ -1523,6 +1523,11 @@ popmotion@9.3.6:
     style-value-types "4.1.4"
     tslib "^2.1.0"
 
+prettier@^3.0.3:
+  version "3.0.3"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.0.3.tgz#432a51f7ba422d1469096c0fdc28e235db8f9643"
+  integrity sha512-L/4pUDMxcNa8R/EthV08Zt42WBO4h1rarVtK0K+QJG0X187OLo7l699jWw0GKuwzkPQ//jMFA/8Xm6Fh3J/DAg==
+
 promise-polyfill@^8.2.3:
   version "8.3.0"
   resolved "https://registry.yarnpkg.com/promise-polyfill/-/promise-polyfill-8.3.0.tgz#9284810268138d103807b11f4e23d5e945a4db63"