Regenerate app icons from Icon Composer (#1005)

* Add Apple Icon Composer source file

Store the .icon project file in design/cmux.icon so the icon can be
edited in Icon Composer and regenerated from source.

* Regenerate all app icons from Icon Composer

Use ictool to render light/dark icon PNGs from the .icon source file
with proper macOS padding. Updates AppIcon, AppIcon-Debug (with DEV
banner), AppIconLight, and AppIconDark imagesets. Adds AppIcon.icon
to the Xcode project as a resource.

* Address review: fix trailing newline, remove .icon from bundle

- Add trailing newline to icon.json files
- Remove AppIcon.icon from Copy Bundle Resources (design-time only)

AppIcon.icon/icon.json

@@ -0,0 +1,35 @@
+{
+  "fill" : "automatic",
+  "groups" : [
+    {
+      "layers" : [
+        {
+          "glass" : false,
+          "image-name" : "cmux-icon-chevron 2.png",
+          "name" : "cmux-icon-chevron 2",
+          "position" : {
+            "scale" : 1,
+            "translation-in-points" : [
+              37.357790031201375,
+              -0.5
+            ]
+          }
+        }
+      ],
+      "shadow" : {
+        "kind" : "neutral",
+        "opacity" : 0.5
+      },
+      "translucency" : {
+        "enabled" : true,
+        "value" : 0.5
+      }
+    }
+  ],
+  "supported-platforms" : {
+    "circles" : [
+      "watchOS"
+    ],
+    "squares" : "shared"
+  }
+}