BaseCollection (and Backbone.Collection) lacked some of the

capabilities available to models and views. It wasn't possible to

provide typed custom options, for example. There were also some mistakes

made with recent changes to specify model types.
This change introduces new generics when subclassing BaseCollection,

allowing an interface to be provided for custom options. This mixes into

an interface (now provided in our version of the Backbone typings) of

standard options for the class. The combined results are passed to

preinitialize, initialize, and constructor. This follows the same

patterns used in Model and View.
Typings for the model on the collection are now fixed. The attempt at

defining and using a Subclass<T> didn't work in practice. We only

needed Class<T>.
The url static attribute can now be a function, which will be called

properly with the instance as this.
The default model type for collections is also now Backbone.Model

instead of BaseModel, to provide better backwards-compatibility.
Unit tests have been added to test all static attribute/function

behavior. Since our testing uses TypeScript, the type-related issues

were also caught by tests.

Unit tests pass.