[FeatureRequest] Markdown support for completion items

[mkrueger]

In VS4Mac we display completion items sometimes differently:

• Obsolete members are strikethrough
• Members show their return type at the right side with a smaller font
• More advanced use cases like the formatting completion show a small description

We're using pango markup for these features - would be really nice if we could get either markdown support or some properties.

ATM we have only these 3 use cases: strikethrough, right side description and small description beside the element. That could be covered with properties alone.

[mkrueger]

Video: https://screencast.com/t/24IKfvePxm

That's how it looks like.

[Pilchie]

Like #16817, but with more scope. Tagging @markw-t, and @olegtk

[sharwell]

• Obsolete members are strikethrough: Moved to Improve completion support for obsolete members #26488
• Members show their return type at the right side with a smaller font: Moved to Show return types in completion #26489

I wasn't sure how to handle the other items above. If I've missed something important, please feel free to open a new issue targeting the specific single feature so we can discuss it. 😄