Improved the initial layout of UPMRs management window.

Change-Id: I26feab03e4fb163a6675d311d0b6ec8f5211c42f

Author: Stewart Miles

source/UnityPackageManagerResolver/src/UnityPackageManagerResolver.cs

@@ -398,6 +398,7 @@ private static void UpdateManifest(ManifestModificationMode mode,
 
                 // Optional when prompting is enabled or forced.
                 var window = MultiSelectWindow.CreateMultiSelectWindow(PLUGIN_NAME);
+                window.minSize = new Vector2(1024, 500);
                 window.AvailableItems = registryItems;
                 window.Sort(1);
                 window.SelectedItems = selectedRegistryUrls;