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 first column and values form the second column. It differs from other renderers that prefer using rows (likely because then the keys will render in bold), but it helps keeping all the information in a Markdown file readable without coming down to horizontal scrolling.

Multi-value (list) entries are rendered as Markdown lists (opposite to one-row tables in other renderers), again, to keep the vertical structure of the block.

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

Demo

Tested scenarios

Use case	Screenshot
Simple key-value block
Single key block
Empty block
List
Single item
Mixed scalar and list values
Block parsing stops on non-front-matter syntax
Block not at document start
List items with colon
Scalar with newlines
Scalar with leading lines
Strip chomping
Keep chomping
Folding
Folding + strip chomping
Folding + keep chomping
Folding + blank lines
Empty scalar
Empty scalar + key
Non-empty scalar + key
Unquote quotes

Typing in

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)