Try WASI in CI

.github/workflows/pr.yaml

@@ -27,6 +27,25 @@ jobs:
       - run: cargo test --verbose --package rayon-core
       - run: ./ci/highlander.sh
 
+  # wasm32-unknown-unknown builds, and even has the runtime fallback for
+  # unsupported threading, but we don't have an environment to execute in.
+  # wasm32-wasi can test the fallback by running in wasmtime.
+  wasm:
+    name: WebAssembly
+    runs-on: ubuntu-latest
+    env:
+      CARGO_TARGET_WASM32_WASI_RUNNER: $HOME/.wasmtime/bin/wasmtime
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: wasm32-unknown-unknown,wasm32-wasi
+      - run: cargo check --verbose --target wasm32-unknown-unknown
+      - run: cargo check --verbose --target wasm32-wasi
+      - run: curl https://wasmtime.dev/install.sh -sSf | bash
+      - run: cargo test --verbose --target wasm32-wasi --package rayon
+      - run: cargo test --verbose --target wasm32-wasi --package rayon-core
+
   fmt:
     name: Format
     runs-on: ubuntu-latest