Request: supporting sub-configs (aka per-directory config)

Is your feature request related to a problem? Please describe.

I like black's default of 88-char line length, except for docs/ renderings, where code blocks may be rendered via Sphinx with 79 char width. This leads to unnecessary scrolling left/right to see the last 9 chars.

It would be useful to enable black to have sub-configurations. This was requested in https://github.com/psf/black/issues/1370, and was postponed into the future.

Describe the solution you'd like

Enabling subdirectories to have their own pyproject.toml, and black will accept overrides from that sub-pyproject.toml.

Describe alternatives you've considered

ruff is a project that adopted sub-configs through use of the extend keyword. It has been great, I have:

• One repo-wide config
• Two sub-configs: docs/ and tests/

This can be an example successful implementation of sub-configs.

Additional context

One counter argument is simplying invoking black once/config:

• Once for the repo root
• Once for the subdirectory with its special config

I think this complicates things (e.g. multiple make commands or multiple pre-commit hooks), though it is a valid workaround

Start by reviewing Black's existing repo-wide pyproject.toml discovery and the prior discussion in issue #1370. Use the mentioned docs/ and tests/ subdirectories, along with Ruff's extend-based configuration as a comparison point. Done means subdirectories can provide pyproject.toml overrides while the root configuration continues to work.