Do not require web argument for serve command when serving RSC apps

[cannikin]

Summary

Right now yarn rw serve assumes there is both a web and api side and so yarn rw serve with no additional arguments will always start both servers. If you try to launch both but either the web or api side is missing, you'll get an error.

However, in RSC-land, there is only one server. I should be able to use yarn rw serve without the web argument as it's the only server that exists anyway.

Detailed proposal

Update checks in packages/cli/src/commands/serve.js to not raise an error if an argument is left off when serving an RSC app.