[JEWEL-1277] Add FrontMatter metadata block support for Markdown

[AlexVanGogen]

Summary

Add support for YAML front matter metadata blocks in Jewel's Markdown rendering. Front matter content (delimited by --- markers at the beginning of a file) is parsed and rendered as a table where keys form the header row and values form the data row. Multi-value (list) entries are rendered as nested tables within cells.

Fixes JEWEL-1277

Changes

• New front-matter extension module (intellij.platform.jewel.markdown.extensions.frontMatter) with a FrontMatterProcessorExtension that integrates with Jewel's Markdown processor pipeline
• Custom block parser (FrontMatterBlockParser) supporting plain key-value pairs, YAML lists, quoted strings, and block scalars (literal | and folded > with all chomping indicators)
• Refactored gfm-tables Table model classes (TableBlock, TableCell, TableHeader, TableRow) from internal to public experimental API to allow reuse by front matter rendering
• Changed TableBlock.header to nullable (TableHeader?) to support headerless tables (used for nested value tables)
• Changed TableCell.content from List<InlineMarkdown> to MarkdownBlock to support block-level content in cells (e.g., nested tables)
• Updated GitHubTableBlockRenderer to render block content in cells via RenderBlock instead of RenderParagraph
• Added 22 unit tests covering various front matter scenarios including block scalars, lists, quoted values, and edge cases

Evidence

Screen.Recording.2026-03-09.at.18.58.35.mov

Release notes

New features

• Added YAML front matter metadata block support for Markdown documents — front matter is rendered as a key-value table ([JEWEL-1277] Add FrontMatter metadata block support for Markdown #3454)