escape hash only with following whitespace

[fominok]

this should fix #509

[kivikakk]

Hi! Thanks for this. Unfortunately, it causes the spec tests to fail; see https://github.com/kivikakk/comrak/actions/runs/12358004998/job/34494314497?pr=510.

That's this example:

Backslash-escaped `#` characters do not count as part
of the closing sequence:

```````````````````````````````` example
### foo \###
## foo #\##
# foo \#
.
<h3>foo ###</h3>
<h2>foo ###</h2>
<h1>foo #</h1>
````````````````````````````````

The # characters on the right-hand side are no longer escaped (because there's no following space), and they're treated as part of the ATX heading a result.

It looks like this may be a more complicated fix; in general, it's hard to say whether or not a character needs to be escaped to be preserved without a lot more knowledge in the formatter.