Treat a code_block markdown token the same as a fence

I believe this should be a fine way to solve this. I don't see anything that
means that a `code_block` is in any way different than a fenced block that
has no syntax specified.

See Textualize#2781.

src/textual/widgets/_markdown.py

@@ -790,7 +790,7 @@ def update(self, markdown: str) -> None:
                             style_stack.pop()
 
                 stack[-1].set_content(content)
-            elif token.type == "fence":
+            elif token.type in ("fence", "code_block"):
                 output.append(
                     MarkdownFence(
                         self,