支持表格单元格合并

[sunsonliu]

Prerequisites

• [ ] There isn't an existing issue that requests the same feature, to avoid duplicates.

Clear and concise description of the problem

• 把支持单元格合并的表格语法放出来
• 支持预览区域表格编辑
• html转Markdown时，支持对合并单元格的表格进行转义

Suggested solution

No response

Further Information

No response

Contributing

Yes, I'd like to submit a PR for this issue.