[markdown] Putting `UnorderedListWithCheckboxSyntax` before `SetextHeaderSyntax` leads to incorrect parsing of `"a\n-"`

[w568w]

(I'm opening a blank issue because the package:markdown issue type isn't available on the New Issue page.)

Problem

If UnorderedListWithCheckboxSyntax precedes SetextHeaderSyntax in BlockParser's blockSyntaxes (as is the case when ExtensionSet.gitHubFlavored is enabled, due to its inclusion of UnorderedListWithCheckboxSyntax), the following text will be rendered as an empty list item:

I am paragraph
-

Actual Output

I am paragraph is ignored, producing an empty list item, as follows:

Expected Output

I am paragraph should be rendered as a SETEXT-style heading. Check:

https://spec.commonmark.org/dingus/?text=I%20am%20paragraph%0A-

Minimal Reproducible Example

import 'package:markdown/markdown.dart' as md;

void main() {
  final md.Document document = md.Document(
    blockSyntaxes: [
      const md.UnorderedListSyntax(), // <-- This line breaks the case
    ],
  );
  final List<md.Node> astNodes = document.parse("I am paragraph\n-");
  print((astNodes[0] as md.Element).tag); // should print: h2; actual print: ul
}

[w568w]

Close in favor of #2108.