PackageToJS: Add selectable UWASI runtime

[krodak]

Overview

Add --wasi-runtime uwasi to PackageToJS while preserving @bjorn3/browser_wasi_shim as the default. The selected runtime flows through template preprocessing, generated browser and Node setup, package metadata, and MiniMake cache fingerprints.

The UWASI adapter uses the public WASI.setInstance and lineBuffered APIs from swiftwasm/uwasi#26. Browser main-thread setup rejects blocking poll_oneoff waits instead of busy-waiting, while browser workers and Node retain UWASI's blocking implementation.

1. Package generation

• Add the WASIRuntime option and CLI parsing.
• Generate the UWASI helper only when selected.
• Preserve existing output when the option is omitted.

2. Adapter behavior

• Delegate instance binding and line buffering to UWASI.
• Preserve PackageToJS's Node extractFile contract.
• Apply the fail-fast sleep policy only when a browser document is present.

3. Tests and documentation

• Type-check browser, Node, and shared-memory template variants.
• Snapshot the UWASI planner graph.
• Exercise only PackageToJS-owned adapter wiring.
• Document the CLI option and generated package shape.

Todo

• feat: add setInstance and a lineBuffered stdio helper for embedders uwasi#26 merged and uwasi@1.6.0 published

Test Plan

• npm ci
• ./Utilities/format.swift
• swift test --package-path ./Plugins/PackageToJS
• node --test Plugins/PackageToJS/Tests/Inputs/UWASIAdapterTests.mjs
• swift format lint --strict Plugins/PackageToJS/Sources/PackageToJS.swift Plugins/PackageToJS/Sources/PackageToJSPlugin.swift Plugins/PackageToJS/Tests/TemplatesTests.swift Plugins/PackageToJS/Tests/PackagingPlannerTests.swift
• git diff --check