feat: implement underline functionality to markdown editor

[penglezos]

Summary

This PR addresses the missing underline formatting option in the markdown editor's visual toolbar. While underline functionality was already available via keyboard shortcut (Ctrl+U), there was no visual button for users who prefer mouse interactions or are unaware of the hotkey.

Test plan

• Start the development server: npm run start
• Navigate to a markdown editor widget
• Add "underline" to the toolbar buttons configuration
• Verify the underline button appears in the toolbar with proper icon
• Test functionality:
  • Click underline button to apply formatting
  • Click again to remove formatting
  • Verify Ctrl+U hotkey still works
  • Confirm strikethrough text renders correctly in preview

Preview

underline.mov

Checklist

• I have read the contribution guidelines.

[martinjagodic]

@penglezos, why is it called horizontal-line instead of underline? I think we should call it for what it is to avoid confusion.

[penglezos]

@penglezos, why is it called horizontal-line instead of underline? I think we should call it for what it is to avoid confusion.

Hey Martin, thanks for the feedback!

Just to clarify, the first PR I opened was for underline, which applies inline text decoration. The second commit that I pushed is a separate feature that inserts a divider (currently named horizontal-line).

You're right that the name could be clearer. I think its better to rename it to horizontal-rule to better reflect its semantic meaning. That should avoid any confusion between actual text underlines and dividers.

[martinjagodic]

Ah, I get it now. Horizontal rule is better, yes. @demshy will review this PR then, he is the master of the editor.

[penglezos]

Ah, I get it now. Horizontal rule is better, yes. @demshy will review this PR then, he is the master of the editor.

Done. Renamed horizontal line to horizontal rule for better clarity. Waiting for the code review.

[martinjagodic]

Thanks @penglezos. I noticed you added clickStrikethroughButton in cypress/support/commands.js, but not for buttons introduced in this PR. Is there a reason for that? I would be great if we had minimal testing for all buttons.