Hide unparsable YAML front matter

[yusufm]

Summary

• advance the preview content offset after syntactic YAML front matter even when YAML parsing fails
• keep front-matter title extraction from assuming the parsed object is always a dictionary
• add regressions for invalid YAML and <title> values inside front matter

Related to #307.

Validation

• xcodebuild test -workspace 'MacDown 3000.xcworkspace' -scheme MacDown -destination 'platform=macOS' -only-testing:MacDownTests/MPStringLookupTests/testFrontMatterContentOffsetWithInvalidYAML -only-testing:MacDownTests/MPRendererEdgeCaseTests/testRendererWithInvalidFrontMatterStillHidesFromPreview -only-testing:MacDownTests/MPRendererEdgeCaseTests/testRendererWithHTMLTitleInFrontMatterStillRendersBody