Fix markdown code-block highlighting

Markdown code blocks should be highlighted as a single block, so set
injection.include-children.

runtime/queries/markdown/injections.scm

@@ -1,6 +1,7 @@
 (fenced_code_block
   (info_string) @injection.language
-  (code_fence_content) @injection.content)
+  (code_fence_content) @injection.content
+  (#set! injection.include-children))
 
 ((html_block) @injection.content
  (#set! injection.language "html"))