Merge cascading/hierarchical configuration files

[kdeldycke]

This depends on #651 to implement the configuration file hierarchy traversal.

This is about merging configuration files at different position in the file system hierarchy.

See:

• https://eslint.org/docs/latest/use/configure/configuration-files#cascading-and-hierarchy
• https://beta.ruff.rs/docs/configuration/#pyprojecttoml-discovery

• click-extra/click_extra/config.py

  Lines 119 to 120 in 6af02e8

  	- ``-C``/``--config`` option, which cannot be used to recursively load another
  	configuration file.

Use cases: mono-repos (and reused workflows?).