refactor(core): update fullscreen methods to return promises

[jansgescheit]

Description

Updated the toggleFullscreen, request, enterElementInFullScreen, and exit methods in VgFullscreenApiService to return Promise<void>. This change ensures consistency and allows for better handling of asynchronous operations related to fullscreen actions.

#146

Checklist

Please, check that you have been followed next steps:

• I've read the (contributing)[/~https://github.com/videogular/ngx-videogular/blob/master/CONTRIBUTING.md] guidelines
• Code compiles correctly (run npm start)
• Created tests (if necessary)
• All tests passing (run npm test)
• Extended the README / documentation (if necessary)

[IxquitilisSaid]

LGTM