Improve UX around "Show solution" button in workshops

[johnpryan]

See this comment for context.

cc: @brianegan

[Spyy004]

Hey, someone please assign me this task. I am happy to contribute to the project by working on this.

I have finished my work and also created a PR of the same. Please have a look : https://github.com/dart-lang/dart-pad/pull/2486

[parlough]

With https://github.com/dart-lang/dart-pad/issues/2808, support for workshops was dropped, so I'll close this as obsolete. If anyone reading this desires any feature similar to workshops, please discuss that in a new issue. Thanks!