Mark PNG icons as maskable

The `maskable` keyword tells Android devices that the icon has been designed with a circular, central safe area, and that Android launchers can
safely crop the edges of the icon instead of shrinking the icon down:
https://css-tricks.com/maskable-icons-android-adaptive-icons-for-your-pwa/

Despite the name, these are not 'masks' in the same way as the Safari icon. The Safari icon is an alpha mask, which is indicated using the
`monochrome` keyword instead.

packages/govuk-frontend/src/govuk/assets/manifest.json

@@ -10,26 +10,29 @@
       "type": "image/svg+xml",
       "sizes": "48x48 72x72 96x96 128x128 256x256"
     },
-    {
-      "src": "images/govuk-icon-mask.svg",
-      "type": "image/svg+xml",
-      "sizes": "48x48 72x72 96x96 128x128 256x256",
-      "purpose": "monochrome"
-    },
     {
       "src": "images/govuk-icon-180.png",
       "type": "image/png",
-      "sizes": "180x180"
+      "sizes": "180x180",
+      "purpose": "maskable any"
     },
     {
       "src": "images/govuk-icon-192.png",
       "type": "image/png",
-      "sizes": "192x192"
+      "sizes": "192x192",
+      "purpose": "maskable any"
     },
     {
       "src": "images/govuk-icon-512.png",
       "type": "image/png",
-      "sizes": "512x512"
+      "sizes": "512x512",
+      "purpose": "maskable any"
+    },
+    {
+      "src": "images/govuk-icon-mask.svg",
+      "type": "image/svg+xml",
+      "sizes": "48x48 72x72 96x96 128x128 256x256",
+      "purpose": "monochrome"
     }
   ]
 }