Md-text format checks: check for headings and lists syntax

[emmapeel2]

It would be good if the markdown format will trigger a failure when heading or lists syntax ( ### or *) does not appear on translation.

I have some placeholder checks for this:

placeholders:r"^\* ":r"^#*\ "

But maybe they should be part of the md-syntax checks.

[nijel]

Are you using native Weblate support for Markdown? In that case, the heading or lists syntax should not be present in a string.

[emmapeel2]

No, I am using a .po generator that reads some markdown files and also some templates in jinja2...

[nijel]

I don't think the block syntax should be exposed to the translator. That's what native support does, and I believe po4a behaves the same on this.