feat(platform): default desktop to use material design

Breaking Change: Previously the default was to use ios mode if a more
specific platform was not determined. Now the material design mode (md)
will apply by default. In most cases this means desktop browsers will
now receive the md mode. Closes #6003

ionic/platform/registry.ts

@@ -9,7 +9,7 @@ const doc: any = document;
 Platform.register({
   name: 'core',
   settings: {
-    mode: 'ios',
+    mode: 'md',
     keyboardHeight: 290,
   }
 });