provide Isabelle/Electron test;

--- a/NEWS	Mon May 09 21:01:12 2022 +0200
+++ b/NEWS	Wed May 11 09:53:29 2022 +0200
@@ -26,6 +26,11 @@
 applications. This essentially provides a freely programmable Chromium
 browser engine that works uniformly on all platforms.
 
+Example:
+
+  URL="https://isabelle.in.tum.de" isabelle electron \
+    --app="$(isabelle getenv -b ISABELLE_HOME)"/src/Tools/Electron/test
+
 
 *** HOL ***
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Tools/Electron/test/main.js	Wed May 11 09:53:29 2022 +0200
@@ -0,0 +1,14 @@
+const { app, BrowserWindow } = require('electron')
+
+const createWindow = () => {
+  const win = new BrowserWindow({
+    width: 800,
+    height: 600
+  })
+
+  win.loadURL(process.env.URL || 'https://isabelle.in.tum.de')
+}
+
+app.whenReady().then(() => {
+  createWindow()
+})

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Tools/Electron/test/package.json	Wed May 11 09:53:29 2022 +0200
@@ -0,0 +1,15 @@
+{
+  "name": "isabelle-electron-test",
+  "version": "1.0.0",
+  "description": "Isabelle/Electron Test",
+  "main": "main.js",
+  "scripts": {
+    "start": "electron ."
+  },
+  "author": "Makarius",
+  "license": "BSD3",
+  "devDependencies": {
+      "electron": "^17.1.2",
+      "@electron/fuses": "1.5.0"
+  }
+}