Add .github/workflows test ensuring correct function

[chadoh]

Let's add a GitHub workflow to the .github/workflows folder that will:

• start with this project
• rm -rf contracts Cargo.toml Cargo.lock
• stellar contract init . (maybe with a few examples?)

I believe this will simulate setting up a new project with stellar contract init some-new-project --frontend-template [this repo]. Please double-check me here!

Then the test can run npm install and npm run build, then use a frontend testing tool to load the index.html page and verify that it loads without any JS errors.

It will be important to have this in place before we get a lot of community-contributed frontend templates, to increase everyone's confidence that a given frontend template will at least load.