chore: migrate eslint to flat config

eslint 9 ignores .eslintrc.* by default, so replace .eslintrc.cjs with a
flat eslint.config.mjs (in the style of graycoreio/daffodil)

eslint.config.mjs

@@ -0,0 +1,29 @@
+import { defineConfig, globalIgnores } from "eslint/config";
+import eslint from "@eslint/js";
+import tseslint from "typescript-eslint";
+
+export default defineConfig([
+  globalIgnores([
+    "**/dist",
+    "**/node_modules",
+    "**/vendor",
+    "_test",
+  ]),
+  {
+    files: ["**/*.ts"],
+    extends: [
+      eslint.configs.recommended,
+      tseslint.configs.recommended,
+    ],
+    rules: {
+      "@typescript-eslint/no-explicit-any": "off",
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        {
+          argsIgnorePattern: "^_",
+          varsIgnorePattern: "^_",
+        },
+      ],
+    },
+  },
+]);