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/types/npm-api.d.ts

@@ -0,0 +1,38 @@
+/**
+ * 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
+ */
+
+// Very incomplete stub with only the methods we use defined.
+declare module 'npm-api' {
+    export default class NpmApi {
+        constructor(...args: any[]);
+
+        list(...args: any[]): any;
+
+        maintainer(...args: any[]): any;
+
+        repo(name: string): Repository;
+
+        reset(...args: any[]): any;
+
+        use(...args: any[]): any;
+
+        view(...args: any[]): any;
+
+    }
+
+    export class Repository {
+        package(): Promise<Package>;
+    }
+
+    export interface Package {
+        name: string;
+        version: string;
+    }
+
+}