add eslint config for the playground JavaScript

Flat config (eslint.config.mjs) plus a minimal package.json exposing an
"npm run lint" script. Declares the browser, third-party (xterm) and
build-generated globals the playground relies on so no-undef stays useful.
node_modules is gitignored.

Author: sylvestre

.gitignore

@@ -1,3 +1,4 @@
 /public/
 /static/syntax-*
 /static/wasm/
+/node_modules/

eslint.config.mjs

@@ -0,0 +1,78 @@
+// Flat ESLint config for the playground's browser-side JavaScript.
+//
+// These files run in the browser (no bundler, no module system) and lean on a
+// handful of globals provided by the page or by third-party scripts loaded via
+// <script> tags (xterm.js, the WASI shim) and by build-generated files
+// (/wasm/commands.js, /wasm/version.js). They are declared below so that
+// no-undef stays useful without flagging those.
+//
+// Run locally with:  npm run lint   (or: npx eslint static/js)
+import js from "@eslint/js";
+
+export default [
+  js.configs.recommended,
+  {
+    // Shared settings for every playground script.
+    files: ["static/js/**/*.js"],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: "script",
+      globals: {
+        // Browser environment
+        window: "readonly",
+        document: "readonly",
+        navigator: "readonly",
+        location: "readonly",
+        console: "readonly",
+        fetch: "readonly",
+        setTimeout: "readonly",
+        clearTimeout: "readonly",
+        URL: "readonly",
+        URLSearchParams: "readonly",
+        CustomEvent: "readonly",
+        TextEncoder: "readonly",
+        TextDecoder: "readonly",
+        WebAssembly: "readonly",
+        SharedArrayBuffer: "readonly",
+        // Third-party globals loaded via <script> on the playground page
+        Terminal: "readonly",
+        FitAddon: "readonly",
+        WebLinksAddon: "readonly",
+        // Generated at build time and injected before these scripts: command
+        // list from /wasm/commands.js, locale and version strings from
+        // /wasm/version.js. Absent in local dev (the code guards with typeof).
+        WASM_COMMANDS: "readonly",
+        WASM_LOCALES: "readonly",
+        UUTILS_WASM_VERSION: "readonly",
+        UUTILS_GREP_VERSION: "readonly",
+        UUTILS_FINDUTILS_VERSION: "readonly",
+        UUTILS_DIFFUTILS_VERSION: "readonly",
+        UUTILS_SED_VERSION: "readonly",
+        SITE_VERSION: "readonly",
+      },
+    },
+    rules: {
+      "no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
+    },
+  },
+  {
+    // playground.js drives the page and calls into the terminal API that
+    // wasm-terminal.js exposes on `window` (loaded as a separate <script>).
+    // These live here rather than in the shared block so they don't clash with
+    // their own definitions in wasm-terminal.js (no-redeclare).
+    files: ["static/js/playground.js"],
+    languageOptions: {
+      globals: {
+        initPlayground: "readonly",
+        runInTerminal: "readonly",
+        loadProgram: "readonly",
+        isProgramLoaded: "readonly",
+        programSize: "readonly",
+        uutilsPrograms: "readonly",
+        getLastCommand: "readonly",
+        setLocale: "readonly",
+        uutilsExecute: "readonly",
+      },
+    },
+  },
+];