ci: Add WebAssembly compilation check

[krodak]

Overview

At PassiveLogic we're adopting CI-level WebAssembly compilation checks across our Swift repositories, so that the WASM build is continuously verified as code evolves (WebAssembly is one of our deployment targets). This PR is a natural extension of that effort to swift-differentiation, requested by @JaapWijnen, and follows the same approach we use elsewhere (e.g. in JavaScriptKit).

It adds a CI job that builds the package for WebAssembly on every pull request, so a change that breaks the WASM build is caught here rather than downstream.

What this verifies

The open-source Swift toolchain's WebAssembly SDK ships the _Differentiation module (_Differentiation.swiftmodule + libswift_Differentiation.a), so #if canImport(_Differentiation) is satisfied on WASM and the differentiation code actually compiles (it is not stubbed out). The existing swift-differentiation-stdlib xcframework dependency stays correctly scoped to macOS/iOS (xcframeworks are Apple-only, and Apple release toolchains do not bundle _Differentiation); on WASM/Linux the toolchain provides the module instead. No Package.swift change is required.

What changed

• .github/workflows/pull_request.yml - new test-wasm job. It runs in the swift:6.3 container, installs the matching WebAssembly Swift SDK via swiftwasm/setup-swiftwasm, and runs swift build --swift-sdk <id> for wasm32-unknown-wasip1.

Tested

• Built locally against the 6.3 WebAssembly SDK: the Differentiation target compiles clean, with the _Differentiation-guarded sources (e.g. Array+Update.swift, Dictionary+Update.swift, Differentiation.swift) actually compiling.

Notes

• Scoped to Swift 6.3 to match the validated toolchain; happy to extend the job to a ["6.2", "6.3"] matrix to mirror the existing Ubuntu test jobs if preferred.