Need a way to provide default values for new launch profiles

In [AzDO 1351857](https://dev.azure.com/devdiv/DevDiv/_workitems/edit/1351857) we encountered a `NullReferenceException` while attempting to set the value of a global launch setting related to IIS/IIS Express. While the handler for this global launch setting should definitely guard against this exception, it is worth noting that with the legacy property pages it was more difficult to get into this situation as WebTools would set default values for various properties (both in a launch profile and in global launch settings) the first time an IIS and/or IIS Express profile was created.

There is no equivalent in the new Launch Profiles UI. We just create an empty profile with the specified command and assume that is good enough. We should devise an extension point for the Launch Profiles UI that allows providers to fill in default values when a new profile is created.