ci: build and lint the wasm32-wasip1 target

Add a job that installs the wasm32-wasip1 target plus the WASI SDK (so the
bundled C in onig_sys can be cross-compiled) and runs cargo build and
clippy -D warnings, so non-Unix compilation can't regress.

.github/workflows/ci.yml

@@ -65,6 +65,29 @@ jobs:
       - run: |
           cargo clippy --all-targets -- -D warnings
 
+  wasm:
+    name: cargo build (wasm32-wasip1)
+    runs-on: ubuntu-latest
+    env:
+      WASI_SDK_VERSION: "33"
+      WASI_SDK_PATH: ${{ github.workspace }}/wasi-sdk
+      # onig_sys compiles bundled C, so it needs a C compiler with a WASI
+      # sysroot to cross-compile to wasm.
+      CC_wasm32_wasip1: ${{ github.workspace }}/wasi-sdk/bin/clang
+      AR_wasm32_wasip1: ${{ github.workspace }}/wasi-sdk/bin/llvm-ar
+    steps:
+      - uses: actions/checkout@v6
+      - name: Install wasm target
+        run: rustup target add wasm32-wasip1
+      - name: Install WASI SDK
+        run: |
+          curl -sSfL "https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-${WASI_SDK_VERSION}/wasi-sdk-${WASI_SDK_VERSION}.0-x86_64-linux.tar.gz" | tar xz
+          mv "wasi-sdk-${WASI_SDK_VERSION}.0-x86_64-linux" "${WASI_SDK_PATH}"
+      - name: Build
+        run: cargo build --target wasm32-wasip1
+      - name: Clippy
+        run: cargo clippy --target wasm32-wasip1 -- -D warnings
+
   grcov:
     name: Code coverage
     runs-on: ${{ matrix.os }}