Add options to modify URL and link element

[twifkak]

This supports prefetching of other sites without revealing the user's IP or cookies to them, using signed exchanges. Proof-of-concept at https://github.com/google/sxg-rs/tree/main/distributor.

The proof-of-concept uses:

• instantUrlModifier to modify the URL only on left-click or prefetch, but preserve the original URL for hover (display at the bottom of the browser) and right-click copy
• instantLinkModifier to add as=document which is necessary to enable the prefetch cache for some specific cases

But I wrote it generically since it may be useful for other use cases.

[dieulot]

Thanks!

Right now instant.page’s goal of fitting in a single TCP packet has no tolerance for experimental/flexible options, but I’m planning to make the code base feature-flaggable through a build step in the next couple of months.