Blockbase: add a page template (#4734)

* Add a page template to blockbase.

* Rename singular to single since we have a page.html now.

blockbase/block-templates/page.html

@@ -0,0 +1,22 @@
+
+<!-- wp:template-part {"slug":"header","tagName":"header"} /-->
+
+<!-- wp:group {"layout":{"inherit":true}} -->
+<div class="wp-block-group">
+<!-- wp:post-title /-->
+</div>
+<!-- /wp:group -->
+
+<!-- wp:group {"tagName":"main"} -->
+<main class="wp-block-group">
+<!-- wp:post-featured-image {"align":"full"} /-->
+
+<!-- wp:post-content {"layout":{"inherit":true}} /-->
+</main>
+<!-- /wp:group -->
+
+<!-- wp:spacer {"height":60} -->
+<div style="height:60px" aria-hidden="true" class="wp-block-spacer"></div>
+<!-- /wp:spacer -->
+
+<!-- wp:template-part {"slug":"footer","tagName":"footer","layout":{"inherit":true},"className":"site-footer-container"} /-->