Alternative storage location for VS classic extension setting

[nguerrera]

In #916 that fixed #660, we had to read .vs/VSWorkspaceSettings.json ourselves when a solution is open. We should keep that for backwards compatibility, but I also think it might make sense to supporting storing this configuration elsewhere.

The problem with .vs/ is that it is commonly .gitignored and contains a lot of temporary junk. Although can do this in .gitignore, it's a pain to figure out.

.vs/
!.vs/VSWorkspaceSettings.json

And so long as we need code to read the data from elsewhere, maybe we should support another location and avoid the .vs/ folder.

And if we're doing that, it becomes interesting to think about whether we can store these settings in a place that works for both VS and VS Code (and potentially other IDEs/editors). We could maybe use cadl-project.yaml, or maybe read .vscode/settings.json in VS classic.

This is now lower priority with #660 fixed, but keeping an issue for things we talked about on #660 but didn't do.