Render GitHub alert blockquotes

[yusufm]

Related to #291

Summary

• Convert GitHub-style blockquote alerts into semantic alert containers after Markdown rendering
• Add bundled alert CSS for preview and styled HTML export
• Add renderer tests for alert conversion and ordinary blockquotes

Tests

• xcodebuild test -workspace 'MacDown 3000.xcworkspace' -scheme MacDown -destination 'platform=macOS' -only-testing:MacDownTests/MPRendererEdgeCaseTests/testRendererFormatsGitHubAlertBlockquote -only-testing:MacDownTests/MPRendererEdgeCaseTests/testRendererLeavesNormalBlockquoteAlone -only-testing:MacDownTests/MPRendererEdgeCaseTests/testPreviewRenderIncludesContentSecurityPolicyAndCheckboxToken

[schuyler]

Thanks for the PR, @yusufm!

Issues

• Nested blockquotes: an alert containing a nested > > quote makes the regex stop at the inner </blockquote>, leaving a dangling outer tag and malformed HTML.
• Single-type test coverage: only WARNING is exercised, so a typo in any of the NOTE/TIP/IMPORTANT/CAUTION class or title mappings would go uncaught.
• Case-insensitivity untested: the case-insensitive flag (which supports [!note]) has no test guarding it against a silent regression.

Suggestions

• A regression test confirming an alert marker inside a fenced code block stays unconverted would guard against the regex becoming too greedy.
• A multi-paragraph alert body test would document the expected behavior and protect the body-capture logic.
• Gating the transform and its stylesheet behind an MPRendererDataSource flag (as other renderer features are) would keep the architecture consistent, though it could also land as a follow-up.
• An NSAssert after the regex compile would surface a pattern failure during development rather than silently turning the transform into a no-op.