Support TOML-based configuration

[TobiasDijkhuis]

TOML files are a convenient way to set the configuration, and would remove the external dependency of ConfigArgParse. Many projects are moving towards it for configuration (see ruff, ty, mypy, for example)

pyproject.toml:

[tool.fprettify]
indent = 4
whitespace-print = true

or fprettify.toml

indent = 4
whitespace-print = true

You can see a simple python implementation from, for example, PyMend

[nbehrnd]

Another example is fortitude.

[TobiasDijkhuis]

@nbehrnd, see #210
I would like to know if you have any comments!