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

PR #268 にリンクされている議論から始め、現在の feature-stubbing の実装を確認して、既存する各 feature と stub の組み合わせをカバーするテストを追加します。生成される prelude と粒度の細かい feature の範囲を決める前に、提案されている作業を issue #250 と比較します。prelude の変更と新しい feature がリグレッションなしで実装され、既存の組み合わせに対するカバレッジが確保されれば完了です。

索引モデルが issue の本文から書いたものです。