Please unblock package markdown-exec

[ Reason ]
The package got rather small upstream modifications since 1.3.0 I'd like
to see in the bookworm release. Another reason is the mark as a core
package which requires a manually unblock by the RT.

An diff about the differences between 1.3.0 and 1.4.0 can be viewed on
the project GitHub website.

https://github.com/pawamoy/markdown-exec/compare/1.3.0...1.4.0

[ Impact ]
The user can use a improved Session handling which was added to 1.4.0.

[ Tests ]
All the tests while package build and within the autopkgtest run
succeeded. Using the newer Debian package in local projects working just
fine.

[ Risks ]
The risks are quite low as only extra functionality was added and no
changes did happen to the existing code.
No issues got reported since the upload of 1.4.0-1 almost 3 weeks ago.

[ Checklist ]
   [x] all changes are documented in the d/changelog
   [x] I reviewed all changes and I approve them
   [x] attach debdiff against the package in testing


[ Other info ]
I'm not aware of any.

unblock markdown-exec/1.4.0-1

Unblocked.