If no IJsRuntime has been added to Services at render time, add a dummy that will throw if requested

[egil]

To make it easier for users to discover the JsRuntime Mock and how/when they need it, I think it would be a great solution to register a dummy IJsRuntime just before the service collection gets turned into a service provider. That dummy should simply throw an exception if every instantiated with a descriptive message to the user about providing i IJsRuntime with links to JsRuntimeMock documentation.

[egil]

Completed by @Siphonophora