[C#] React to Workspace Folder Changes

[tsmaeder]

In #14223, we found out that C# plugin works reads workspace roots at statup, but it does not handle changing workspace roots. We would need that support in order to merge the fix for #13427.

So this task is about reacting to didChangeWorkspaceFolders events and "do the right thing" (to be defined)

[tsmaeder]

So the items to implement here would be:

1. if a new "solution" is detected, pop up the "solution picker UI" (see [C#] Handle Multiple Solutions #14370) with the existing solution pre-selected
2. of the currently open solution is removed from the workspace, offer to choose another solution
3. when the last one is gone, stop the language server.