text: Add a foundation for a block-based WYSIWYG editor

[EnoroF]

• Add the foundation for a block-based WYSIWYG Markdown editor.
• Reuse existing Markdown parsing, rendering, selection, and undo history.
• Support basic formatting, IME, list indentation, task lists, and read-only mode.
• Add an editor example using the original Markdown demo content and plugins.
• Keep images, tables, code blocks, and custom elements visible but not directly editable.
• Validated on Windows: 165 text tests and Clippy passed.