[SUGGESTION] Support Multi-root workspaces

Describe the feature you'd like

Yes, it's not a great thing to do, but sometimes one ends up with related solutions (.sln files) in the same repository, without one solution to rule them all.

in this case, a multi-root workspace looks like a good idea to see them in the same place.

This works well for other extensions BTW. For example, Azure Functions can only support one Azure Functions project at a time, but if you use multi-root workspace, you can have multiple Azure Functions project, each in one root folder of this workspace, and it works fine.

But if you try to do this for dotnet, for example, by adding a .vscode/settings.json file to every root folder, you get:
This setting cannot be applied in this workspace. It will be applied when you open the containing workspace folder directly.

So, the request is to make this work, and allow us to have one default solution per multi-root workspace folder.

Alternatives considered

Probably adding another solution that includes all the projects from all the folders in the multi-root workspace. But this is not great as it doesn't show which is which. Especially if there are no shared projects between the solutions (which is the more suitable scenario for multi-root workspaces).

Environment Information

No response

Start by examining how the C# Dev Kit handles .vscode/settings.json, solution discovery, and workspace folders; the issue provides no specific file or test entry point. Done means a multi-root workspace can use one default solution per root folder without the current setting warning.