Support different options for empty line mode in mark down

[JiuqingSong]

Current markdown parser will always ignore empty lines. But actually if there are multiple empty lines, we should keep one.

This change introduces 3 empty line modes:

• preserve: Preserve empty lines, unless it is right next to list, quote, table
• remove: Remove all empty lines (current behavior)
• merge: If there are multiple empty lines, keep one (default)

[Copilot]

Pull Request Overview

This PR introduces configurable empty line handling modes in the markdown parser. Previously, the parser always removed empty lines, but now users can choose between three modes: preserve, remove, or merge (default). This change provides more control over how empty lines are processed when converting markdown to content model.

• Added a new MarkdownOptions interface with configurable empty line handling modes
• Updated the markdown processor to support three empty line modes: preserve, remove, and merge
• Enhanced the demo UI with radio buttons to test different empty line modes

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
MarkdownOptions.ts	Defines new options interface with empty line handling configuration
markdownProcessor.ts	Implements empty line processing logic for all three modes
convertMarkdownToContentModel.ts	Updates API to accept options parameter with backward compatibility
markdownProcessorTest.ts	Adds comprehensive test suites for each empty line mode
convertMarkdownToContentModelTest.ts	Updates test to use new options parameter
MarkdownPane.tsx	Adds demo UI for testing different empty line modes

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