eslint: Type-check ESLint configuration.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -105,6 +105,7 @@
     "@types/blueimp-md5": "^2.18.0",
     "@types/co-body": "^6.1.3",
     "@types/confusing-browser-globals": "^1.0.3",
+    "@types/eslint-config-prettier": "^6.11.3",
     "@types/gtag.js": "^0.0.20",
     "@types/is-url": "^1.2.32",
     "@types/jquery": "^3.3.31",
@@ -125,6 +126,7 @@
     "@types/winchan": "^0.2.0",
     "@typescript-eslint/eslint-plugin": "^8.2.0",
     "@typescript-eslint/parser": "^8.2.0",
+    "@typescript-eslint/utils": "^8.25.0",
     "astro-eslint-parser": "^1.2.2",
     "babel-plugin-istanbul": "^7.0.0",
     "callsites": "^4.2.0",
@@ -170,6 +172,7 @@
   },
   "pnpm": {
     "overrides": {
+      "@types/eslint": "-",
       "source-map@^0.6": "npm:source-map-js@^1.2.1"
     },
     "patchedDependencies": {