Adapt the URL open command to the platform

With this new version we use $OSTYPE bash env var to query what OS
are we running on.

- On Linux, ex: Fedora, we use `xdg-open`
- On Windows, ex: msys, we use `start`
- For other other variants we try to use `open`

We should update this script when we notice that `open` is not
available on a particular platform.

build_demo_web.sh

@@ -32,4 +32,13 @@ wasm-bindgen "target/wasm32-unknown-unknown/$BUILD/$TARGET_NAME" \
 
 echo "Finished: docs/${CRATE_NAME}.wasm"
 
-open http://localhost:8888/index.html
+if [[ "$OSTYPE" == "linux-gnu"* ]]; then
+  # Linux, ex: Fedora
+  xdg-open http://localhost:8888/index.html
+elif [[ "$OSTYPE" == "msys" ]]; then
+  # Windows
+  start http://localhost:8888/index.html
+else
+  # Darmin aka MacOS or something else
+  open http://localhost:8888/index.html
+fi