Public programmatic API

[akaihola]

Similar to Black in psf/black#292, Darker should probably define which parts of the Python API are public and guaranteed to be stable according to semantic versioning.

Currently many classes and functions used across module borders inside Darker are not prefixed with an underscore, which could lead developers to assume they belong to a public stable API. However, for many of those code objects it probably doesn't make sense, and instead they should be considered private and internal.

One option is to prefix all functions, classes and methods save a few public ones with underscores, and tweak linter and IDE options to silence warnings about importing them across Darker modules.

The other option is to define the public API in documentation and/or code comments instead.

See #214 for the origin of this discussion.