Analyze function.json and .ps1 at FunctionLoadTime to prevent issues

[TylerLeonhardt]

See this TODO:
https://github.com/Azure/azure-functions-powershell-worker/blob/dev/src/Azure.Functions.PowerShell.Worker/Function/FunctionLoader.cs#L22

It would be nice if we can:

• Analyze the .ps*1 to make sure it has the right parameters for each input binding
• Make sure the ps*1 exists
• Make sure the entry point exists if defined

[TylerLeonhardt]

We could possibly check the AST for the parameter names - that would be safe