Fix HTML injection within markdown (helix-editor#5265)

HTML nodes should be combined injections in the markdown block
grammar. When nodes are together the highlighting works properly but
when there is markdown content between HTML nodes like in a `<details>`
tag, the highlighting of the closing tag breaks since tree-sitter-html
looks for opening and closing tags.

runtime/queries/markdown/injections.scm

@@ -5,7 +5,10 @@
     (language) @injection.language)
   (code_fence_content) @injection.content (#set! injection.include-unnamed-children))
 
-((html_block) @injection.content (#set! injection.language "html") (#set! injection.include-unnamed-children))
+((html_block) @injection.content
+ (#set! injection.language "html")
+ (#set! injection.include-unnamed-children)
+ (#set! injection.combined))
 
 ((pipe_table_cell) @injection.content (#set! injection.language "markdown.inline") (#set! injection.include-unnamed-children))