Add additional checking for Azure Functions restrictions in Static Web Apps

[anthonychu]

Currently, there are some restrictions for Static Web Apps functions that are beyond those of regular function apps.

• Core Tools should be the latest v3 version
• Only Node 12, .NET 3.1, and Python 3.8 are supported
• Only HTTP triggers are supported
• Only valid routePrefix in host.json is api
• It's not good practice to nest the API folder inside the web app. Not sure if we want to have a warning here. There's potential for exposing the API code if it's deployed as static files too, and npm modules in the function app can cause the static app to exceed the 4000 file limit

Internal tracking link: https://dev.azure.com/azuretracking/Azure%20tracking/_workitems/edit/1678

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