Add schema for local.settings.json

Originally filed by @burkeholland in https://github.com/microsoft/vscode-azurefunctions/issues/1811 who wanted to make it easier to add CORS settings. We have a schema (which gives us autocomplete in VS Code) for other files like proxies.json/function.json/host.json and would be great to have the same for local.settings.json

cc @jeffhollan