Ensure all consumers of @edx/frontend-build > v12.9.2 have serve NPM script

[adamstankiewicz]

See https://github.com/openedx/frontend-build/blob/master/docs/0003-fedx-scripts-serve.md for more details.

Now that the fedx-scripts supports a serve command, each consuming micro-frontend should be able to add a serve script to their package.json NPM scripts:

{
    "scripts": {
        "serve": "fedx-scripts serve"
    }
}

Assumes the consuming application is running at least v12.9.2 of @edx/frontend-build.