Make `SimpleConfigModel` available via public API

[vlada-shubina]

trafficstars

Consider making SimpleConfigModel and loading from json public.

It would allow template-analyzer tool to do more advanced checks (https://github.com/sayedihashimi/dotnet-new-web/issues/42), now the tool parses json directly.

It might be useful for other tools that might need to read content on template.json file. Probably we should consider better naming as SimpleConfigModel doesn't sound like a good name (TemplateConfigModel may be better).

[sayedihashimi]

+1