Add Typescript Declarations

[okerx]

The package has no corresponding Typescript declarations, therefore it cannot be used in a TS project.

[doubleforte]

+1.

[dealancer]

There is a workaround I used to make it work in TS.

1. Add "allowJs": true into tsconfig.json
2. Add following declarations into photoswipe-dynamic-caption-plugin.d.ts:

declare module 'photoswipe-dynamic-caption-plugin';

3. Add following declarations into photoswipe.d.ts:
   declare module 'photoswipe';
   declare module 'photoswipe/lightbox';

Also don't forget to include css.