JS: Add ESLint config

To use this: Create package.json and run:
npm install --save-dev eslint @eslint/js

eslint.config.mjs

@@ -0,0 +1,26 @@
+// prepend adminer/include/functions.js to adminer/include/editing.js and editor/include/editing.js, then delete
+
+import { globalIgnores } from "eslint/config";
+import js from "@eslint/js";
+import globals from "globals";
+
+export default [
+	globalIgnores(["externals/"]),
+	js.configs.recommended,
+	{
+		languageOptions: {
+			globals: {
+				...globals.browser,
+				jush: false, jushLinks: false,
+				offlineMessage: false, thousandsSeparator: false, // include/design.inc.php
+				indexColumns: false, // select.inc.php
+				tablePos: false, em: false, // schema.inc.php
+			}
+		},
+		rules: {
+			"no-var": "error",
+			"prefer-const": "error",
+			"no-unused-vars": "off", //! we want this only on global level
+		},
+	},
+];