Split speculation-rules into separate features for prefetch and prerender

[jgraham]

We are evaluating speculation rules as separate work items for prefetch and prerender. However they seem to both correspond to a single speculation-rules feature at the moment.

Given that there's high implementation complexity here it seems unlikely that both features will be shipped simultaneously, so we should split the web-feature early so that we can correctly associate what's in planning/development with a specific web feature.

[ddbeck]

This is also a candidate for a "composite" feature, such that a speculation-rules is made up of prefetch and prerender. See #971 for more.