Add improved markdown front matter rendering

[mjbvz]

Fixes #316140

Let's try with table as the default initially but we can switch to hide is we prefer

[Copilot]

Pull request overview

Adds built-in YAML front matter (“preamble”) support to the Markdown preview, replacing the previous dependency on markdown-it-front-matter with an in-repo markdown-it extension and exposing a new preview setting to control rendering style.

Changes:

• Introduces a new markdown-it block rule + renderer for YAML front matter with hide, codeBlock, and table rendering styles.
• Adds markdown.preview.frontMatter setting (default: table) plus CSS styling and localized configuration strings.
• Adds engine tests covering rendering modes, invalid YAML handling, and non-front-matter cases.

Show a summary per file

File	Description
extensions/markdown-language-features/src/util/dom.ts	Adds escapeHtml helper for safely rendering YAML-derived content.
extensions/markdown-language-features/src/typings/ref.d.ts	Removes leftover typing shim for markdown-it-front-matter.
extensions/markdown-language-features/src/test/engine.test.ts	Adds tests for front matter rendering behaviors.
extensions/markdown-language-features/src/preview/previewConfig.ts	Tracks new preview.frontMatter setting for preview configuration change detection.
extensions/markdown-language-features/src/markdownEngine.ts	Switches to the new built-in YAML preamble extension.
extensions/markdown-language-features/src/extensions/yamlPreamble/yamlPreamble.ts	Implements parsing + rendering of YAML front matter (hide/codeBlock/table + error rendering).
extensions/markdown-language-features/package.nls.json	Adds localized strings for the new setting and enum options.
extensions/markdown-language-features/package.json	Adds markdown.preview.frontMatter configuration and replaces dependency with yaml.
extensions/markdown-language-features/package-lock.json	Updates lockfile: removes markdown-it-front-matter, adds yaml.
extensions/markdown-language-features/media/markdown.css	Adds styles for front matter table/code block and error rendering.

Copilot's findings

Files not reviewed (1)

• extensions/markdown-language-features/package-lock.json: Language not supported

• Files reviewed: 8/10 changed files
• Comments generated: 1