Fix typing of Stack for TypeScript 3.9.2

[nechutny]

Hello, when using pnotify with project in typesciprt 3.9.2 I've got error

[at-loader] ./node_modules/@pnotify/core/Stack.d.ts:121:21 
    TS2709: Cannot use namespace 'Notice' as a type. 

[at-loader] ./node_modules/@pnotify/core/Stack.d.ts:130:20 
    TS2709: Cannot use namespace 'Notice' as a type. 

[at-loader] ./node_modules/@pnotify/core/Stack.d.ts:142:25 
    TS2709: Cannot use namespace 'Notice' as a type. 

[at-loader] ./node_modules/@pnotify/core/Stack.d.ts:149:14 
    TS2709: Cannot use namespace 'Notice' as a type. ```

So I've prepared fix for this - use proper typeof in Stack.d.ts

[irving-caamal]

these seem very useful, up.

[SeanPlane-dev]

I'm experiencing this issue too. This fix would be greatly appreciated