Allow assembly-level [IdeSettings]

[bricelam]

It's currently only allowed on test methods and classes. It would be nice to specify assembly-wide values:

[assembly: IdeSettings(RootSuffix = "Test")]

[sharwell]

I've generally worked around this by having a common base class. For example:
https://github.com/dotnet/roslyn/blob/main/src/VisualStudio/IntegrationTest/New.IntegrationTests/AbstractIntegrationTest.cs

It seems reasonable to allow usage of the attribute on an assembly if it doesn't cause other problems.