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

.github/workflows/nodejs-pkg.yml

@@ -7,17 +7,20 @@ jobs:
 
     runs-on: ubuntu-latest
 
+    env:
+      pkg-directory: ./desktop/pkg
+
     steps:
     - uses: actions/checkout@v1
     - uses: actions/setup-node@v1
       with:
         node-version: '12.x'
     - name: install
-      working-directory: pkg
+      working-directory: ${{env.pkg-directory}}
       run: yarn
     - name: test
-      working-directory: pkg
+      working-directory: ${{env.pkg-directory}}
       run: yarn test
     - name: run
-      working-directory: pkg
+      working-directory: ${{env.pkg-directory}}
       run: bin/run