Use blankline to delimit html block in markdown colorization

**Bug**
Issue #7725

Nested html is not colorized properly for markdown:

```md
<div class="custom-class" markdown="1">
  <div>
    nested div
  </div>
</div>
```

**Fix**
According to the commonmark spec, general html blocks end with a blank line: http://spec.commonmark.org/0.25/#html-blocks

This change relaxes the while so that we only check for blanklines, instead of trying to match the start tag (which fails when nesting elements)

extensions/markdown/syntaxes/markdown.tmLanguage

@@ -309,7 +309,7 @@
 								</dict>
 							</array>
 							<key>while</key>
-							<string>\G(?!&lt;/\2\s*&gt;)</string>
+							<string>^(?!\s*$)</string>
 						</dict>
 					</array>
 				</dict>