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

scripts/setup-env.sh

@@ -12,7 +12,7 @@ main () {
 
   source "$ROOT_DIR/xplat/js/env-utils/setup_env_vars.sh"
 
-  export SONAR_DIR="$ROOT_DIR/xplat/infinity"
+  export SONAR_DIR="$ROOT_DIR/xplat/sonar"
   export PATH="$SONAR_DIR/node_modules/.bin:$ROOT_DIR/xplat/third-party/node/bin:$ROOT_DIR/xplat/third-party/yarn:$PATH"
 }