Add speculation rules prefetch

This ports the relevant text from https://wicg.github.io/nav-speculation/speculation-rules.html, with minor improvements and additions. It does *not* port the actual navigational prefetching algorithms, which are left in https://wicg.github.io/nav-speculation/prefetch.html for now.

Notable changes from that document:

* Allows treating prerender rules as prefetch rules.

* Allows the presence of target_hint (a prerender-specific speculation rule key), but ignores it.

* Follows the precedent set by import maps a bit more closely in how it integrates with the script processing model. Normatively, it fires error events on the script element for the few things that cause complete parsing failures.

* Adds conformance requirements for speculation rules JSON documents, following the precedent set by import maps.

* Adds explicit callouts for when the implementation may report warnings for parsing failures, following the precedent set by import maps.

* Fixes a few minor bugs in the parsing algorithms. E.g., the previous text did not correctly deal with how CSS selector parsing returned a list, or called the slightly wrong algorithm for parsing URL patterns.

* Simplified the processing model for the Speculation-Rules HTTP header. The previous text added various concepts around pending fetches and periodic implementation-defined processing of those fetches, all designed to allow the implementation to de-prioritize these fetches in favor of other things that happen during loading. This version instead explicitly allows an implementation-defined wait time and uses a separate task source which user agents can deprioritize.

* Be explicit about when to consider speculative loads, instead of using vague language about "whenever something relevant changes".

* Add re-parsing of <script type=speculationrules> on base URL changes, to match the Chromium implementation.

Author: domenic