Export Preloader to use preloadURL dynamically

[julianrubisch]

Why?

I am currently testing the preloader's capabilities and found that it could be beneficial to call preloadURL dynamically, for example

• using an IntersectionObserver, like https://github.com/GoogleChromeLabs/quicklink, for example.
• using mouseover events, like https://instant.page/
• using MutationObservers, (e.g. in Stimulus controllers) that listen for a certain data attribute to be set,
• or even in other custom web components

[julianrubisch]

heads up, I've put up an alternative here: #911