Add support for SSR (as client-side library)

[budnix]

The PR adds SSR support as a client-side library. These changes involve:

• Removed direct references to the window, and document objects. They are still there but are used after module initialization;
• Added the export key in the package.json file to support ESM, CJS, and UMD builds;
• Added GH workflow for tests;
• Removed code that tried to detect the moment module as it was not compatibe with ESM. Instead, I added a new useMoment method that can be used to tell the Pikaday to use it;
• Added TS types;