[markdown] Fix setext heading precedence over lists in gitHubFlavored

[hkarmoush]

Summary

ExtensionSet.gitHubFlavored's block syntax list checked UnorderedListWithCheckboxSyntax before SetextHeaderSyntax. Since custom document.blockSyntaxes are always tried before the standard ones in BlockParser.parseLines, a lone -/= line continuing a paragraph was parsed as an empty list item instead of a setext heading underline — unlike the default parser (which orders SetextHeaderSyntax first) and ExtensionSet.gitHubWeb (which already puts SetextHeaderWithIdSyntax before its list syntaxes).

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

void main() {
  final doc = md.Document(blockSyntaxes: [const md.UnorderedListSyntax()]);
  print((doc.parseLines('I am paragraph\n-')[0] as md.Element).tag);
  // actual: ul   expected: h2
}

This matches CommonMark / GitHub's own rendering (confirmed in the issue by @lrhn) — see https://spec.commonmark.org/dingus/?text=I%20am%20paragraph%0A-.

Fixes #2108 (duplicate of #1953).

Changes

• Adds SetextHeaderSyntax to ExtensionSet.gitHubFlavored's block syntax list, ordered before the list syntaxes.
• Adds a regression test in test/regression_test.dart.
• Adds a changelog entry under the unreleased 7.4.0 section.

Test plan

• dart test — full markdown suite passes (2763 passed, 1 pre-existing skip, 0 failures)
• dart analyze --fatal-infos . — no issues
• dart format --output=none --set-exit-if-changed on changed files — no changes needed

[gemini-code-assist]

Code Review

This pull request fixes an issue in ExtensionSet.gitHubFlavored where a lone - or = line continuing a paragraph was incorrectly parsed as an empty list item instead of a setext heading underline. This is resolved by adding SetextHeaderSyntax before the list syntaxes in the block syntaxes list. A regression test has been added to verify this behavior. There are no review comments, and I have no additional feedback to provide.