Configure linter + prettier.

- This is based on existing expectations for TS code in Google-esc repos.
- First part of the change (we have not run any linter or formatting commands). After this changeset goes in I'll do a mass changeset push.

Fixes https://b.corp.google.com/issues/411384603

package.json

@@ -2,12 +2,25 @@
   "name": "gemini-code",
   "version": "1.0.0",
   "private": true,
+  "type": "module",
   "workspaces": [
     "packages/*"
   ],
   "scripts": {
     "build": "npm run build --workspaces",
     "test": "npm run test --workspaces",
-    "start": "npm run start --workspace=gemini-code-cli"
+    "start": "npm run start --workspace=gemini-code-cli",
+    "lint": "eslint .",
+    "format": "prettier --write ."
+  },
+  "devDependencies": {
+    "eslint": "^9.24.0",
+    "eslint-config-prettier": "^10.1.2",
+    "eslint-plugin-import": "^2.31.0",
+    "eslint-plugin-react": "^7.37.5",
+    "eslint-plugin-react-hooks": "^5.2.0",
+    "globals": "^16.0.0",
+    "prettier": "^3.5.3",
+    "typescript-eslint": "^8.30.1"
   }
 }