Build for arm64 on macOS.

Fixes #1076.

For now we only build a DMG and zip for arm64, because the arm64 pkg
would overwrite the x64 pkg
(https://github.com/electron-userland/electron-builder/issues/5847).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -53,9 +53,14 @@
     "mac": {
       "category": "public.app-category.productivity",
       "target": [
-        "dmg",
+        {
-        "pkg",
+          "target": "default",
-        "zip"
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        },
+        "pkg"
       ],
       "darkModeSupport": true,
       "artifactName": "${productName}-${version}-${arch}.${ext}",