Whitespace between comments is no longer removed

In #184, processing was updated to be non-destructive. A side effect of this code change is that whitespace between comments is no longer removed:

import markdownify

md = markdownify.MarkdownConverter().convert

md("""
<p>line 1</p>
<!-- comment -->
<!-- comment -->
<!-- comment -->
<!-- comment -->
<p>line 2</p>""")
# 'line 1\n\n\n\n\n\nline 2'

This occurs because the code that conditionally omits whitespace looks at the adjacent siblings on both sides to check if they are block elements. Because comments are no longer deleted, this check sees comments instead of block elements.

Start with the whitespace-handling logic that checks adjacent siblings, using the MarkdownConverter().convert example in the issue to reproduce the extra newlines between comments. Compare the behavior introduced by #184 and verify that the converted output no longer contains the unwanted whitespace between the comment nodes.