static type stubs

This package lacks static typing stubs and py.typed (empty) marker file.

```
Skipping analyzing "caseconverter": module is installed, but missing library stubs or py.typed marker  [import-untyped]
```

Also refactor the tests into separate /tests/ folder outside of the package. Add \_\_init\_\_.py, conftest.py files. Rename test files, `test_` prefix.