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

從 PR #268 中連結的討論開始，接著檢查目前的 feature-stubbing 實作，並新增涵蓋每個現有 feature 與 stub 組合的測試。在決定產生的 prelude 與細粒度 feature 的範圍之前，將提議的工作與 issue #250 進行比較。完成的標準是：prelude 的變更與新 feature 在沒有回歸的情況下實作完成，且現有組合都有涵蓋。

由索引模型根據 Issue 內容生成。