Move desktop-related code to "desktop" subfolder (#872)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/872
Move all the JS code related to desktop app to "desktop" subfolder.

The structure of "desktop" folder:
- `src` - JS code of Flipper desktop app executing in Electron Renderer (Chrome) process. This folder also contains all the Flipper plugins in subfolder "src/plugins".
- `static` - JS code of Flipper desktop app bootstrapping executing in Electron Main (Node.js) process
- `pkg` - Flipper packaging lib and CLI tool
- `doctor` - Flipper diagnostics lib and CLI tool
- `scripts` - Build scripts for Flipper desktop app
- `headless` - Headless version of Flipper app
- `headless-tests` - Integration tests running agains Flipper headless version

Reviewed By: passy

Differential Revision: D20249304

fbshipit-source-id: 9a51c63b51b92b758a02fc8ebf7d3d116770efe9

desktop/static/transforms/electron-stubs.js

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+const babylon = require('@babel/parser');
+const fs = require('fs');
+
+const electronStubs = babylon.parseExpression(
+  fs.readFileSync('static/electron-stubs.notjs').toString(),
+);
+
+module.exports = function(babel) {
+  return {
+    name: 'replace-electron-requires-with-stubs',
+    visitor: {
+      CallExpression(path) {
+        if (
+          path.node.type === 'CallExpression' &&
+          path.node.callee.type === 'Identifier' &&
+          path.node.callee.name === 'require' &&
+          path.node.arguments.length > 0
+        ) {
+          if (path.node.arguments[0].value === 'electron') {
+            path.replaceWith(electronStubs);
+          }
+        }
+      },
+    },
+  };
+};