eslint: Migrate to flat configuration.

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

package.json

@@ -97,11 +97,16 @@
   "devDependencies": {
     "@babel/eslint-parser": "^7.11.3",
     "@babel/plugin-transform-modules-commonjs": "^7.19.6",
+    "@eslint/eslintrc": "^2.1.4",
+    "@eslint/js": "^8.57.1",
     "@formatjs/cli": "^6.0.0",
     "@giphy/js-types": "^5.1.0",
     "@types/autosize": "^4.0.1",
     "@types/blueimp-md5": "^2.18.0",
     "@types/co-body": "^6.1.3",
+    "@types/confusing-browser-globals": "^1.0.3",
+    "@types/eslint__eslintrc": "^2.1.2",
+    "@types/eslint__js": "^8.42.3",
     "@types/gtag.js": "^0.0.20",
     "@types/is-url": "^1.2.32",
     "@types/jquery": "^3.3.31",
@@ -139,6 +144,7 @@
     "eslint-plugin-import": "^2.22.0",
     "eslint-plugin-no-jquery": "^3.0.2",
     "eslint-plugin-unicorn": "^56.0.0",
+    "globals": "^13.24.0",
     "jsdom": "^25.0.0",
     "mockdate": "^3.0.2",
     "nyc": "^17.0.0",
@@ -152,6 +158,7 @@
     "svgo": "^3.0.0",
     "swagger-parser": "^10.0.0",
     "typescript": "^5.0.2",
+    "typescript-eslint": "^8.13.0",
     "vnu-jar": "^24.10.17",
     "webpack-dev-server": "^5.0.2",
     "xvfb": "^0.4.0",