add icon for Xcode beta version

Xcodes/Frontend/XcodeList/XcodeListViewRow.swift

@@ -73,7 +73,7 @@ struct XcodeListViewRow: View {
         if let icon = xcode.icon {
             Image(nsImage: icon)
         } else {
-            Image("xcode")
+            Image(xcode.version.isPrerelease ? "xcode-beta" : "xcode")
                 .resizable()
                 .frame(width: 32, height: 32)
                 .opacity(0.2)

Xcodes/Resources/Assets.xcassets/xcode-beta.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "filename" : "Image.png",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}