Migrate ESLint to v10 flat config, add diff override

Author: Copilot

.eslintrc.js

@@ -0,0 +1,21 @@
+const js = require("@eslint/js");
+const globals = require("globals");
+
+module.exports = [
+    js.configs.recommended,
+    {
+        languageOptions: {
+            ecmaVersion: 2017,
+            globals: {
+                ...globals.node,
+                ...globals.es2017
+            }
+        },
+        rules: {
+            "indent": ["error", 4],
+            "linebreak-style": ["error", "unix"],
+            "semi": ["error", "always"],
+            "no-cond-assign": ["error", "always"]
+        }
+    }
+];