Add emscripten build

This closes shader-slang#5115.

CMakePresets.json

@@ -15,6 +15,16 @@
         "CMAKE_MSVC_RUNTIME_LIBRARY": "MultiThreaded$<$<CONFIG:Debug>:Debug>"
       }
     },
+    {
+      "name": "emscripten",
+      "description": "Emscripten-based Wasm build",
+      "generator": "ninja",
+      "binaryDir": "${sourceDir}/build.em",
+      "cacheVariables": {
+        "SLANG_SLANG_LLVM_FLAVOR": "DISABLE",
+        "CMAKE_EXE_LINKER_FLAGS": "-sASSERTIONS -sALLOW_MEMORY_GROWTH"
+      }
+    },
     {
       "name": "msvc-base",
       "hidden": true,
@@ -74,6 +84,14 @@
       "configurePreset": "default",
       "configuration": "Release"
     },
+    {
+      "name": "emscripten",
+      "configurePreset": "emscripten",
+      "configuration": "Release",
+      "targets": [
+        "slang"
+      ]
+    },
     {
       "name": "generators",
       "inherits": "release",

docs/building.md

@@ -47,6 +47,20 @@ cmake --build --preset release # to build from the CLI
 
 The `vs2022-dev` preset turns on features that makes debugging easy.
 
+### WebAssembly build
+
+First do a native build of the `all-generators` target, so that you get the generator executables in a path like `build\generators\Debug\bin` under the
+Slang source tree.
+
+For an [emscripten](https://emscripten.org) -based build targeting [WebAssembly](https://webassembly.org), run:
+```bash
+emcmake cmake -DSLANG_GENERATORS_PATH=%SLANG_SRC_PATH%\build\generators\Debug\bin --preset emscripten -G "Ninja"
+cmake --build --preset emscripten --target slang
+```
+
+**Note:** `emcmake` is a `cmake` wrapper that comes with [`emsdk`](https://emscripten.org/docs/getting_started/downloads.html), it will wrap your `cmake` calls with setup options that add support for emscripten.
+**Note:** If this fails, try running the command that `emcmake` outputs, directly.
+
 ## Testing
 
 ```bash