Add a WebAssembly build to release (WebAssembly#6351)

Simply build wasm-opt with Emscripten and bundle that up.

Example build:

https://github.com/kripken/binaryen/releases/tag/wasm-build-1

Specifically

binaryen-wasm-build-1-wasm.tar.gz

Only 1.72 MB, as it's just wasm-opt and not any other tool, so it is
much smaller than our other targets. Perhaps we will add more of the
tools later as needed (wasm-metadce, wasm-split, etc.).

Also update the readme regarding which toolchains use us as a library, that I
noticed while editing it to add the release platforms.

.github/workflows/create_release.yml

@@ -168,3 +168,69 @@ jobs:
         files: |
           ${{ steps.archive.outputs.tarball }}
           ${{ steps.archive.outputs.shasum }}
+
+  # Build using Emscripten to JavaScript+WebAssembly.
+  build-node:
+    name: node
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
+    steps:
+    - uses: actions/setup-python@v1
+      with:
+        python-version: '3.x'
+    - uses: actions/checkout@v1
+      with:
+        submodules: true
+    - name: install ninja
+      run: sudo apt-get install ninja-build
+    - name: emsdk install
+      run: |
+        mkdir $HOME/emsdk
+        git clone --depth 1 https://github.com/emscripten-core/emsdk.git $HOME/emsdk
+        $HOME/emsdk/emsdk update-tags
+        $HOME/emsdk/emsdk install tot
+        $HOME/emsdk/emsdk activate tot
+    - name: update path
+      run:  echo "PATH=$PATH:$HOME/emsdk" >> $GITHUB_ENV
+
+    # Configure with wasm EH and pthreads for maximal performance.
+    - name: cmake
+      run: |
+        source $HOME/emsdk/emsdk_env.sh
+        emcmake cmake -S . -B out -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=out/install -DEMSCRIPTEN_ENABLE_WASM_EH=ON -DEMSCRIPTEN_ENABLE_PTHREADS=ON
+
+    # Build wasm-opt for now TODO add other tools as desired
+    - name: build
+      run: ninja -C out wasm-opt
+
+    # Minimal smoke test: roundtrip a file.
+    # TODO: Add more testing here, but the full test suite is overkill as there
+    #       is a 0.5 second cost to each run of wasm-opt.js
+    - name: test
+      run: |
+        node out/bin/wasm-opt.js test/hello_world.wat --print > out/t.wat
+        diff test/hello_world.wat out/t.wat
+
+    - name: archive
+      id: archive
+      run: |
+        VERSION=$GITHUB_REF_NAME
+        PKGNAME="binaryen-$VERSION-node"
+        TARBALL=$PKGNAME.tar.gz
+        SHASUM=$PKGNAME.tar.gz.sha256
+        mkdir binaryen-$VERSION
+        cp out/bin/wasm-opt* binaryen-$VERSION/
+        tar -czf $TARBALL binaryen-$VERSION
+        cmake -E sha256sum $TARBALL > $SHASUM
+        echo "::set-output name=tarball::$TARBALL"
+        echo "::set-output name=shasum::$SHASUM"
+
+    - name: upload tarball
+      uses: softprops/action-gh-release@v1
+      with:
+        draft: true
+        files: |
+          ${{ steps.archive.outputs.tarball }}
+          ${{ steps.archive.outputs.shasum }}

README.md

@@ -24,7 +24,18 @@ effective**:
    wasm [minification], similar to minification for JavaScript, CSS, etc., all
    of which are language-specific.
 
-Compilers using Binaryen include:
+Toolchains using Binaryen as a **component** (typically running `wasm-opt`) include:
+
+  * [`Emscripten`](http://emscripten.org) (C/C++)
+  * [`wasm-pack`](https://github.com/rustwasm/wasm-pack) (Rust)
+  * [`J2CL`](https://j2cl.io/) (Java; [`J2Wasm`](https://github.com/google/j2cl/tree/master/samples/wasm))
+  * [`Kotlin`](https://kotl.in/wasmgc) (Kotlin/Wasm)
+  * [`Dart`](https://flutter.dev/wasm) (Flutter)
+
+For more on how some of those work, see the toolchain architecture parts of
+the [V8 WasmGC porting blogpost](https://v8.dev/blog/wasm-gc-porting).
+
+Compilers using Binaryen as a **library** include:
 
  * [`AssemblyScript`](https://github.com/AssemblyScript/assemblyscript) which compiles a variant of TypeScript to WebAssembly
  * [`wasm2js`](https://github.com/WebAssembly/binaryen/blob/main/src/wasm2js.h) which compiles WebAssembly to JS
@@ -387,6 +398,26 @@ Binaryen.js can be built using Emscripten, which can be installed via [the SDK](
 
    CMake generates a project named "ALL_BUILD.vcxproj" for conveniently building all the projects.
 
+## Releases
+
+Builds are distributed by the various toolchains that use Binaryen, like
+Emscripten, `wasm-pack`, etc. There are also official releases on GitHub:
+
+https://github.com/WebAssembly/binaryen/releases
+
+Currently builds of the following platforms are included:
+
+ * `Linux-x86_64`
+ * `Linux-arm64`
+ * `MacOS-x86_64`
+ * `MacOS-arm64`
+ * `Windows-x86_64`
+ * `Node.js` (experimental): A port of `wasm-opt` to JavaScript+WebAssembly.
+   Run `node wasm-opt.js` as a drop-in replacement for a native build of
+   `wasm-opt`, on any platform that Node.js runs on. Requires Node.js 18+ (for
+   Wasm EH and Wasm Threads). (Note that this build may also run in Deno, Bun,
+   or other JavaScript+WebAssembly environments, but is tested only on Node.js.)
+
 ## Running
 
 ### wasm-opt