feat: loadPrevNext module - preloading images

[Carnoisseur1979]

This implements a loadPrevNext module, to preload next and/or previous images when lazy loading, by a specified amount.

This replaces the functionality lost when the lazy module was removed in v9. See discussion #6350 .

[nolimits4web]

Thanks!

I think this shouldn't be a module, but part of the core.

• unlazy and preload functions can go here https://github.com/nolimits4web/swiper/blob/master/src/shared/process-lazy-preloader.js
• core parameter lazyPreloadPrevNext that accepts number (default is 0)
• instead of hooking into init and slideChange events just call preload into the places these events are emitted

[Carnoisseur1979]

Thanks!

I think this shouldn't be a module, but part of the core.

* `unlazy` and `preload` functions can go here https://github.com/nolimits4web/swiper/blob/master/src/shared/process-lazy-preloader.js

* core parameter `lazyPreloadPrevNext` that accepts number (default is 0)

* instead of hooking into `init` and `slideChange` events just call `preload` into the places these events are emitted

I agree - as soon as I posted this I had second thoughts about it being a module!
I was also thinking that it should account for rewind mode - looping round when needed.

I'll have a go in another PR.

[Carnoisseur1979]

@nolimits4web see second attempt, as part of the core, in #6544