Previously, the LibWeb bindings generator would output multiple per
interface files like Prototype/Constructor/Namespace/GlobalMixin
depending on the contents of that IDL file.
This complicates the build system as it means that it does not know
what files will be generated without knowledge of the contents of that
IDL file.
Instead, for each IDL file only generate a single Bindings/<IDLFile>.h
and Bindings/<IDLFile>.cpp.
The Cache and CacheStorage interfaces are quite intertwined. A Cache
object can only be created through CacheStorage, so the Cache interface
cannot be tested on its own. So adding just this stub will allow
implementing the CacheStorage interfaces that create Cache objects. We
can then implement the Cache interface piecemeal.
