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/pkg/README.md

@@ -0,0 +1,70 @@
+# flipper-pkg
+
+`flipper-pkg` is a **work-in-progress** tool for bundling and publishing
+Flipper plugins.
+
+<!-- toc -->
+* [Usage](#usage)
+* [Commands](#commands)
+<!-- tocstop -->
+# Usage
+<!-- usage -->
+```sh-session
+$ npm install -g mycli
+$ mycli COMMAND
+running command...
+$ mycli (-v|--version|version)
+mycli/0.0.0 darwin-x64 node-v12.14.0
+$ mycli --help [COMMAND]
+USAGE
+  $ mycli COMMAND
+...
+```
+<!-- usagestop -->
+# Commands
+<!-- commands -->
+* [`mycli hello [FILE]`](#mycli-hello-file)
+* [`mycli help [COMMAND]`](#mycli-help-command)
+
+## `mycli hello [FILE]`
+
+describe the command here
+
+```
+USAGE
+  $ mycli hello [FILE]
+
+OPTIONS
+  -f, --force
+  -h, --help       show CLI help
+  -n, --name=name  name to print
+
+EXAMPLE
+  $ mycli hello
+  hello world from ./src/hello.ts!
+```
+
+_See code: [src/commands/hello.ts](https://github.com/passy/mycli/blob/v0.0.0/src/commands/hello.ts)_
+
+## `mycli help [COMMAND]`
+
+display help for mycli
+
+```
+USAGE
+  $ mycli help [COMMAND]
+
+ARGUMENTS
+  COMMAND  command to show help for
+
+OPTIONS
+  --all  see all commands in CLI
+```
+
+_See code: [@oclif/plugin-help](https://github.com/oclif/plugin-help/blob/v2.2.3/src/commands/help.ts)_
+<!-- commandsstop -->
+
+
+## License
+
+[MIT](LICENSE)