DidChangeConfigurationParams should contain a workspace URI

With 3.6.0 the protocol specification got multi-workspace support, however the workspace/didChangeConfiguration notification is still specced for single workspace roots. Often there is project/workspace specific configuration which should also trigger the workspace/didChangeConfiguration notification, however the server has no way of knowing for which workspace this new configuration applies and could just apply it to all workspaces which may yield in wrong future behaviour of the server.

Adding the workspace URI to DidChangeConfigurationParams is an easy way to make servers aware of workspace specific configuration without breaking backwards compatibility. If the field is unset/empty the server can just treat it like it did before such a change and if the server doesn't even support it, it will just treat it like before.