More granular features for stubbing

During a recent PR that patched a bug in feature-based WASI interface stubbing, the point of how to deal with outgoing-handler came up:

https://github.com/bytecodealliance/ComponentizeJS/pull/268#discussion_r2223776655

It *seems* like there are at least two solutions that could improve the ergonomics of stubbing:

- stubbing out `fetch()` itself via generated prelude to delete fetch
- more granular features to enable stubbing

Implementing this and avoiding introducing bugs probably requires a bunch of upfront work:

- Adding tests for all the current features & their usage (making sure existing stub combinations work)
- implementing the prelude changes
- adding & implementing the new granular features

Some of this work may overlap (and possibly resolve) #250