Add WASM build target for Web version of glslang

This adds build rules to support generating a WASM binary to be used
on the web. The API exposed to web applications is definated in the
new glslang.js.cpp file.

HOWTO-build-wasm.md

@@ -0,0 +1,14 @@
+Build as usual (see `README.md`), but:
+  * Need `emsdk` in your *PATH*
+    + Instructions located
+      [here](https://emscripten.org/docs/getting_started/downloads.html#sdk-download-and-install)
+  * Do not checkout SPIRV-Tools into External/
+    + Does not work correctly with emscripten out of the box and we don't want
+      it in the build anyway.
+    + *TBD* Add flag to disable SPIRV-Tool inclusion even if directory is
+      present.
+  * Wrap call to `cmake` using `emconfigure`:
+    + e.g. `emconfigure cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_GLSLANG_WEB=ON
+    -DENABLE_HLSL=OFF -DCMAKE_INSTALL_PREFIX="$(pwd)/install" ..`
+    + To get a 'true' size, make sure to use `brotli` to compress the .js and .wasm files
+