Refactor template file structure

[Fleker]

• [ ] Migrate all template files to subfolders based on language (ie. templates/python)
• [ ] Rename (lang).jinja2 to base.(lang).jinja2
• [ ] Update all import paths

This will declutter the existing cluttered directory, which has a wide variety of file extensions and help new people on the project navigate and make sense of what files are what. A file like clang.jinja2 makes sense to me, but is itself not very descriptive.