Allow installing multiple themes via URL, as required by parent/child themes. (#2581)

## Motivation for the change, related issues

At present it isn't possible to install Child themes via the playground
through the URL without using a blueprint.

## Implementation details

Iterate over the theme parameter.

## Testing Instructions (or ideally a Blueprint)

Compare
http://127.0.0.1:5400/website-server/?theme=acai&theme=skincare-brand&url=wp-admin/themes.php
to
http://playground.wordpress.net/?theme=acai&theme=skincare-brand&url=wp-admin/themes.php

Author: dd32

packages/playground/website/src/lib/state/url/resolve-blueprint-from-url.ts

@@ -102,15 +102,21 @@ export async function resolveBlueprintFromURL(
 							url: query.get('import-site')!,
 						},
 					} as StepDefinition),
-				query.get('theme') &&
-					({
-						step: 'installTheme',
-						themeData: {
-							resource: 'wordpress.org/themes',
-							slug: query.get('theme')!,
-						},
-						progress: { weight: 2 },
-					} as StepDefinition),
+				...query.getAll('theme').map(
+					(theme, index, themes) =>
+						({
+							step: 'installTheme',
+							themeData: {
+								resource: 'wordpress.org/themes',
+								slug: theme,
+							},
+							options: {
+								// Activate only the last theme in the list.
+								activate: index === themes.length - 1,
+							},
+							progress: { weight: 2 },
+						} as StepDefinition)
+				),
 			].filter(Boolean),
 		};
 		source = {