Make a WASM build of Themes Playground

[rydmike]

To make a WASM build of Themes Playground requires remove the package url_launcher and all it's usage, as mentioned here https://docs.flutter.dev/platform-integration/web/wasm#requires-preview-js-interop-to-access-browser-and-js-apis.

Could still be interesting to make a branch and try it out.
I could also use it try and see if some of the bugs I've seen on JS builds, that do not exist on VM builds, goes away when using WASM.

Still on the fence, would be cool to try, but a bit lazy remove all the URL links in the code. Might as well wait until WASM support using URL links in a web app.