Support partial serverless.template

It is difficult to mange the serverless.template as a single file, it becomes cumberosme to navigate and find the appropriate sections. Including long strings e.g. database queries or emails directly in the JSON requires escaping line endings and quotes, this makes them difficult to edit in place.

Allow multiple serverless.template files (or a precursor file type) to be created and then combine these into a single template on publish. Add a substitute function that allows strings to be pulled from a co-located .txt file, escaping quotes and line endings.

Start by tracing how serverless.template is read and combined during publish; the issue names no implementation files or tests. Review the existing template handling and determine the supported structure for multiple files and co-located .txt substitutions. Done means publishing produces one valid combined template and correctly escapes substituted strings.

Written by the indexing model from the issue text.