Don't allow backslash + newline to affect block structure.

Note that as a result of this change, the following,
which formerly produced a header with two lines separated
by a line break, will now produce a header followed by a
paragraph:

    # Hi\
    there

This may affect some existing documents that relied on
this undocumented and unintended behavior.

This change makes pandoc more consistent with other
Markdown implementations, and with itself (since the two-space
version of a line break doesn't work inside ATX headers, and
neither version works inside Setext headers).

Closes #3730.

test/command/3730.md

@@ -0,0 +1,21 @@
+````
+% pandoc
+nice line\
+```
+code
+```
+^D
+<p>nice line<br />
+</p>
+<pre><code>code</code></pre>
+````
+
+```
+% pandoc
+# hi\
+there
+^D
+<h1 id="hi">hi<br />
+</h1>
+<p>there</p>
+```