Add "MacOS" to supportedDevices (#3648)

Summary:
This is already supported, when running Flipper on Mac, so just need update schema.

One use case for `MacOS`, is to have a static plugin available to users before they launch any app/device. So this can be used as Welcome plugin, Documentation or any other "static" plugin not requiring any other device except the one running Flipper Desktop.

## Changelog

- add `MacOS` to supported devices

Pull Request resolved: https://github.com/facebook/flipper/pull/3648

Test Plan: - Script `lint` should pass.

Reviewed By: antonk52

Differential Revision: D35810577

Pulled By: lblasa

fbshipit-source-id: e1a2948fdf67535d28b68eeab649214b44ebcf9c

desktop/pkg/schemas/plugin-package-v2.json

@@ -62,9 +62,9 @@
         "type": "object",
         "properties": {
           "os": {
-            "description": "Device OS: iOS, Android or Metro. Lack of this property means that all OSes supported.",
+            "description": "Device OS: iOS, Android, Metro, MacOS, Linux, Windows. Lack of this property means that all OSes supported.",
             "type": "string",
-            "enum": ["iOS", "Android", "Metro"]
+            "enum": ["iOS", "Android", "Metro", "MacOS", "Linux", "Windows"]
           },
           "type": {
             "description": "Device type: physical or emulator or dummy. Lack of this property means it supports physical, emulator and dummy devices.",