feat(preview2-shim): implement browser HTTP fetch support via JSPI

[danbopes]

Implement the browser WASI HTTP shim using the fetch() API with JSPI for async suspension, replacing the previous non-functional stubs.

• Rewrite http.js with Fields, OutgoingRequest, IncomingResponse, IncomingBody, FutureIncomingResponse, and outgoing-handler.handle()
• Fix Pollable in io.js: add ready(), block(), subscribe() support
• Fix clocks.js: subscribeDuration(0) yields via setTimeout, fix subscribeInstant this-binding for standalone function calls
• Fix blockingRead double-reader bug on ReadableStream
• Add browser tests ported from wasmtime p2_* suite (clocks, poll, HTTP)
• Replace hand-written WIT deps with canonical versions via wkg

[vados-cosmonic]

This is fantastic work @danbopes , thanks for contributing so much implementation to the browser preview2-shim, it has not gotten any attention for a long time.

I have a few nitpicks/small changes proposed, please take a look and let me know if you have any questions/disagree!

[vados-cosmonic]

Hey @danbopes thanks for your hard work here and iterating so fast and explaining stuff to me where necessary.

If I can ask for one more thing -- can you either format your commits to match the conventional commits standard or squash to one commit with a title like the following:

feat(p2-shim): implement browser HTTP fetch support via JSPI

The tagging is picked up by the automation in-repo, and we don't squash actually -- we fast-forward so that we can (easily) catch non-conforming commit titles in CI

[danbopes]

@vados-cosmonic Done

[jco-release-bot]

🎉 The code in this PR was made available in the following release:
https://github.com/bytecodealliance/jco/releases/tag/preview2-shim-v0.17.8