Support relative paths in stylesheets

[mattiasb]

Description

Resolve style sheets in markdown-css-paths relative to the Markdown file

This fixes loading of "./stylesheet.css" from #834.

Related Issue

#855

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[mattiasb]

Rebased and used the correct issue number (#855) in CHANGES.md.

[mattiasb]

Using concat over file-name-concat seems like an okay workaround for Emacs 27 FWIW.

But I don't know if one can rely on default-directory always ending with a /.

EDIT: Nevermind, I'm stupid. Since it's in a unit test we control all in data so we don't need to bother with this hypothetical.