Make XDT support that Xrm.Navigation.navigateTo() can navigate to default dashboard

[jacobblom]

The API method Xrm.Navigation.navigateTo() supports sending the user to a dashboard, the default if nothing else is specified, which happens to be exactly what I need but cannot achieve with the current version of XDT.

The method is documented here:
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/xrm-navigation/navigateto
and navigating to a dashboard is documented as this:

All that is needed from XDT is to extend PageType in xrm.d.ts from

type PageType = "entitylist" | "webresource";

to

type PageType = "entitylist" | "webresource" | "dashboard";