Standalone --- throws a front matter exception instead of rendering a thematic break #28

Reproduction

With tempest/markdown 1.2.2 on PHP 8.5.9:

$markdown = new \Tempest\Markdown\Markdown(null);
echo $markdown->parse('---')->html;

Actual behavior: Tempest\Markdown\Exceptions\FrontMatterWasNotProperlyClosed is thrown.

Expected behavior: a standalone --- is a Markdown thematic break, so parsing should succeed and render an <hr> element (CommonMark emits <hr />).

FrontMatterRule::shouldParse() currently treats every document starting with --- as front matter, including a standalone thematic-break marker. It may need to require a following newline/content and a closing delimiter before claiming this input, or allow the thematic-break rule to handle the standalone case.