Add initial isolated world spec

spec/fixtures/api/isolated-preload.js

@@ -0,0 +1,15 @@
+const {ipcRenderer} = require('electron')
+
+window.foo = 3
+
+window.addEventListener('message', (event) => {
+  ipcRenderer.send('isolated-world', {
+    preloadContext: {
+      preloadProperty: typeof window.foo,
+      pageProperty: typeof window.hello,
+      typeofRequire: typeof require,
+      typeofProcess: typeof process
+    },
+    pageContext: event.data
+  })
+})