Allow display prop on markdown span style

[yoavbls]

Style attribute added to span tags in #97793
Currently, it allows setting color and background-color but many times the background color doesn't apply because span is display: inline by default and doesn't have an actual width and height.
Adding display prop to the allowed styles will fix that and also will open up many other (safe) options for layoutting.

e.g, in pretty-ts-errors I had to use codicon classes to apply background-color and make things inlined. That classes bring unwanted styles like user-select: none that prevent selecting and copying the content.
Now it's a huge deal for tens of thousands of developers that can't copy the types from their errors.

I know that no one wants to keep adding styles to the sanitized regex and it opens the barrier for other requests like this but display prop is different. It is not a niche CSS prop, it is one of the most essential props in CSS and one of the few things that markdown can't do.

I made a pull request that add it, I hope it helps: #180498

[yoavbls]

Hi there!
I just wanted to mention that JetBrains released their alternative solution
and their code blocks are copyable.

We don't want to stay behind, It could be so nice if you'll approve this little line in my PR 🙏🏼
https://github.com/microsoft/vscode/pull/180498/files

[VSCodeTriageBot]

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Happy Coding!