Add support for TOML configuration files (`.importlinter.toml` and `importlinter.toml`)

[chirizxc]

Currently, only pyproject.toml is supported, which can be a bit inconvenient as it can be too large.

The problem with the .importlinter file is that it does not have highlighting, which is also not very convenient.

Most Python tools support separate files in the toml format: uv (uv.toml), ruff (ruff.toml), pytest from version 9.0.0 onwards.

[seddonym]

Yeah great point. I'll comment on the PR separately but I agree we should add support for importlinter.toml.