fix: shift headings with commonmark AST

[hsjobeki]

This PR fixes the issue, that the current headings are shifted line wise without taking the context into account.

Leading to issues that shift single line comments in code blocks # comment because they look the same as # heading

We should use a real commonmark library because code blocks have multiple variants and can also be nested. I am also not sure if there are any other edge cases where a line starts with # and not beeing headings. Any regex or other self-cooked algoritms are not desireable due to complexity and maintenance overhead.

Nobody noticed this issue yet; i guess. But as we progress with the migration it is rather ugly and should be fixed by now.

closes #113

Done:

• [x] Bump fix version
• [x] Add/change unit tests

[infinisil]

(needs a rebase)

[infinisil]

This seems to have broken the Nixpkgs manual build: https://github.com/NixOS/nixpkgs/pull/302990#issuecomment-2049471406