Updated to eslint 9

This isn't officially compatible with the oldest version of Node that
Jasmine supports, but it works. If it stops working, we can always disable
linting in CI builds on older Node versions.

eslint.config.mjs

@@ -0,0 +1,54 @@
+import { defineConfig } from "eslint/config";
+import globals from "globals";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all
+});
+
+export default defineConfig([{
+  extends: compat.extends("plugin:compat/recommended"),
+
+  languageOptions: {
+    globals: {
+      ...globals.browser,
+      ...globals.node,
+    },
+
+    ecmaVersion: 2018,
+    sourceType: "commonjs",
+  },
+
+  rules: {
+    curly: "error",
+
+    quotes: ["error", "single", {
+      avoidEscape: true,
+    }],
+
+    "no-unused-vars": ["error", {
+      args: "none",
+    }],
+
+    "no-implicit-globals": "error",
+    "block-spacing": "error",
+    "func-call-spacing": ["error", "never"],
+    "key-spacing": "error",
+    "no-tabs": "error",
+    "no-trailing-spaces": "error",
+    "no-whitespace-before-property": "error",
+    semi: ["error", "always"],
+    "space-before-blocks": "error",
+    "no-eval": "error",
+    "no-var": "error",
+    "no-debugger": "error",
+    "no-console": "error",
+  },
+}]);