Handle Multiline headings properly

test/djot-to-html/003-headings/expected

@@ -15,5 +15,13 @@
     <p>Even more content</p>
     <h1>Back up</h1>
     <p>Some final content</p>
+    <h1>A Multiline Heading</h1>
+    <h2>A Level 2 Multiline Heading</h2>
+    <p>## Heading ### Not a heading</p>
+    <p>### Heading ## Not a heading</p>
+    <h1>A Multiline Heading</h1>
+    <h2>A Level 2 Multiline Heading</h2>
+    <h1>A Multiline Heading</h1>
+    <h2>A Level 2 Multiline Heading</h2>
   </body>
 </html>

test/djot-to-html/003-headings/test.dj

@@ -26,3 +26,27 @@ Even more content
 # Back up
 
 Some final content
+
+# A Multiline
+# Heading
+
+## A Level 2
+## Multiline Heading
+
+## Heading
+### Not a heading
+
+### Heading
+## Not a heading
+
+# A Multiline
+Heading
+
+## A Level 2
+Multiline Heading
+
+# A Multiline
+  Heading
+
+## A Level 2
+   Multiline Heading