Add the ability to open a solution file

[JoeRobich]

An experiment to see what it would be like if opening a loose solution file in VSCode would then open the containing folder and launch roslyn lsp against the solution.

• If workspaceFolders are empty and the C# extension starts with the solution file as the active document, then we open the parent folder in the current window.
• If workspacesFolders are not empty and the active document changes to a solution file outside the workspacesFolders, then we open the parent folder in a new window with the solution file open.
• When RoslynLanguageServer is determining the initial solution to open, we check whether the active file is a solution file and use that instead of any settings.

Downsides:
We don't want to activate C# outside of C# scenarios, since solution file are used for languages other than C# (for example C++). One thought is that a Solution Launcher extension could be made that always activate on startup and contained the code in solutionFileWorkspaceHandler.ts. Users could then opt-in to the solution loading behavior by installing this extension.

From within VS Code:

Screen.Recording.2026-02-02.at.10.02.23.PM.mov

From the command line:

Screen.Recording.2026-02-02.at.10.05.04.PM.mov

[JoeRobich]

This doesn't work as I expected and the C# extension doesn't load when only a loose solution file is opened in VS Code.

Suggested change

	"workspaceContains:**/*.{csproj,csx,cake,sln,slnx,slnf}"
	"workspaceContains:**/*.{csproj,csx,cake}"