Add DMG target to mac distribution (#833)

Summary:
Fixes https://github.com/facebook/flipper/issues/807 by adding a DMG target to distribute the application for mac.

The `.dmg` file is `97.2 MB` while the `.zip` is `101.5 MB`. 😄

![image](https://user-images.githubusercontent.com/6207220/75275890-7c32b480-5805-11ea-9d05-9a4f47d03562.png)

## Changelog

Add DMG target for mac distribution
Pull Request resolved: https://github.com/facebook/flipper/pull/833

Test Plan:
1. `yarn build --mac`;
1. Check `flipper-mac.dmg` inside of `dist`.

 ---

**The background is completely discussable, I borrowed it [from an old project](https://github.com/lucasbento/react-native-actions/tree/feature/electron/packages/electron-react-native-actions) of mine and it's [totally not inspired by WebStorm `.dmg` file](https://user-images.githubusercontent.com/6207220/75276024-b3a16100-5805-11ea-8c69-063fe28316c1.png).**

Reviewed By: nikoant

Differential Revision: D20139188

Pulled By: passy

fbshipit-source-id: 1aaaabd7c6ac51d12865388bf962d164f80a7a14

package.json

@@ -25,6 +25,27 @@
         "NSUserNotificationAlertStyle": "alert"
       }
     },
+    "dmg": {
+      "background": "dmgBackground.png",
+      "icon": "icon.icns",
+      "iconSize": 155,
+      "window": {
+        "width": 660,
+        "height": 400
+      },
+      "contents": [
+        {
+          "x": 123,
+          "y": 172
+        },
+        {
+          "x": 539,
+          "y": 168,
+          "type": "link",
+          "path": "/Applications"
+        }
+      ]
+    },
     "win": {
       "publisherName": "Facebook, Inc.",
       "sign": null

scripts/build-release.ts

@@ -67,7 +67,7 @@ async function buildDist(buildFolder: string) {
   const postBuildCallbacks: (() => void)[] = [];
 
   if (process.argv.indexOf('--mac') > -1) {
-    targetsRaw.push(Platform.MAC.createTarget(['dir']));
+    targetsRaw.push(Platform.MAC.createTarget(['dir', 'dmg']));
     postBuildCallbacks.push(() =>
       spawn('zip', ['-qyr9', '../Flipper-mac.zip', 'Flipper.app'], {
         cwd: path.join(__dirname, '..', 'dist', 'mac'),