Add slash "/" commands to editor

Add slash commands to format text:

/bold: Makes the selected text bold
/italic: Makes the selected text italicized
/underline: Underlines the selected text
/code: Marks the selected text as code
/strikethrough: Adds a strikethrough to the selected text

https://app.tablane.net/1073/Tablane/Roadmap/63931a801406f1a0bc562f02