Editor: Fix get_block_templates() to get templates for a post type.

When a post type is passed to `get_block_templates()` in the query, return only the templates that match that post type.

Fixes an issue where:
* when a block template provided by a theme has been edited by the user
* and that template has specific defined `postTypes`
* but after editing, the template was available for all post types.

Follow-up to [52062].

Props aljullu, bernhard-reiter, hellofromTonya.
Fixes #55881.

git-svn-id: https://develop.svn.wordpress.org/trunk@54184 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/block-theme/templates/custom-single-post-template.html

@@ -0,0 +1,3 @@
+<!-- wp:paragraph -->
+<p>Custom Single Post template</p>
+<!-- /wp:paragraph -->

tests/phpunit/data/themedir1/block-theme/theme.json

@@ -58,6 +58,11 @@
 		{
 			"name": "page-home",
 			"title": "Homepage template"
+		},
+		{
+			"name": "custom-single-post-template",
+			"title": "Custom Single Post template",
+			"postTypes": ["post"]
 		}
 	],
 	"templateParts": [