Code generation templates support

Code generation is not flexible now.
1. Only concrete languages (Swift, TypeScript, Flow) can be specified.
2. The structure of the generated code cannot be changed.
3. New language support requires changes directly in the code.

So I propose to introduce templates support.

Great example of code generation is [SwiftGen](https://github.com/SwiftGen/SwiftGen). It uses a template language syntax similar to Django and Mustache. It has default templates and allows creating custom templates.

There's js library [mustache.js](https://github.com/janl/mustache.js/) that provides templates support.

My proposal is to migrate code generation into _mustache.js-like_ templates for all supported languages.

No implementation files or tests are named. Start by locating the existing TypeScript code-generation entry points and reviewing how Swift, TypeScript, and Flow output is produced; done means generation supports customizable templates and new language support without direct generator-code changes.

Written by the indexing model from the issue text.