Jss: TypeScript declaration of classes should be class instead of interface

[mfellner]

__Expected behavior:__
Inside index.d.ts the declaration of classes (e.g. Renderer, SheetsRegistry, etc.) should be declare class instead of interface.

__Describe the bug:__
When using jss in TypeScript, some types cannot be instantiated because they are declared as interface instead of class. Example:

'SheetsRegistry' only refers to a type, but is being used as a value here.

__Codesandbox link:__
https://codesandbox.io/s/m4p166n59y

__Versions (please complete the following information):__

• jss: 10.0.0-alpha.7

[HenriBeck]

Thanks, will be fixed this evening.

[mfellner]

@HenriBeck I had already prepared a PR, feel free to use it. Cheers!