feat: add Insert Link to Header in File command for markdown

[yogeshwaran-c]

What kind of change does this PR introduce?

Feature — adds a new command to the markdown-language-features extension.

What is the current behavior?

When writing markdown links to headers (e.g., [text](#header-name)), users must type the slugified header name with dashes instead of spaces in the suggest widget. There is no way to search for headers using their natural text with spaces.

Closes #174999

What is the new behavior?

Adds a new Markdown: Insert Link to Header in File command (markdown.editor.insertHeaderLink) that:

1. Retrieves all headers from the current document via vscode.executeDocumentSymbolProvider
2. Shows a quick pick with all headers listed by their natural text (e.g., "A header with spaces in the title")
3. Displays the corresponding slug in the description (e.g., a-header-with-spaces-in-the-title)
4. Uses VS Code's fuzzy matching so users can search by natural text with spaces
5. Inserts a properly formatted markdown link [Header Text](#slug) at the cursor position
6. If text is selected, uses the selection as the link text instead

This follows the approach suggested by @mjbvz in the issue: a dedicated command with a quick pick UI, similar to how Markdown: Insert Link to File in Workspace works.

Implementation details

• New InsertHeaderLink class in insertResource.ts, following the same Command pattern as InsertLinkFromWorkspace and InsertImageFromWorkspace
• Uses the existing ISlugifier (GitHub-compatible) to generate correct slugs, including handling of duplicate headers
• Recursively flattens nested DocumentSymbol hierarchy to collect all headers
• Inserts via SnippetString with a placeholder for the link text when no text is selected

Additional context

The command is available in the Command Palette as "Markdown: Insert Link to Header in File" whenever a markdown file is active in the editor.