Updates to border image generator tool

[schalkneethling]

What was incorrect, unhelpful, or incomplete?

This stems from the discussion I started concerning the tools on MDN Web Docs. Please read the discussion for more details and context.

There are a couple of common problems between these tools and issues filed here. This issue will track the work on addressing the relevant issues and do a bit of a rewrite of the border image generator.

Known issues

• possible Border-image generator UI updates #96
• MDN Border-image generator: Move border-image-source to the end in CSS Code output #93
• [Border-image generator] border-image-slice fill keyword not toggled in CSS output #63
• [Border-image generator] include gradient as border-image-source example #60

Common issues

• Ensure that the tool is responsive and usable on as many platforms and devices as possible.
• Rewrite the JS to use modern syntax without introducing the need for polyfills

The work will be split up into two to three separate pull requests. The first will address the responsiveness, the second the JS, and the last any existing issues that were not already fixed by the JS updates.

Reviewers for the work will be between @caugner and @LeoMcA

What did you expect to see?

The tool is responsive and usable across viewports and devices. The code has been updated and is in a better state to ensure they can be more easily maintained going forward.

Do you have any supporting links, references, or citations?

https://github.com/orgs/mdn/discussions/674

Do you have anything more you want to share?

No response