fix(config): search for config file recursively on --config-path

[rami3l]

Note

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

LLM has been used to analyze the existing usage of directory walking behavior.
The implementation of this patch is otherwise fully manual.

Closes #4660 based on the design discussed in #4660 (comment).

Background

The --config-path help description says:

        --config-path [Path for the configuration file]
                        Recursively searches the given path for the
                        rustfmt.toml config file. If not found reverts to the
                        input file path

However this description is not quite appropriate because:

• It's unclear whether the path being passed in is a directory, or a TOML file.
• Recursive search doesn't really happen when this argument is passed to rustfmt.

To be more precise, the current semantics of --config-path is that when it receives a path, it chooses to enter one of the following modes in the below fallback order:

• Base dir mode: If path is exists and metadata says it's a dir, then search for predefined names ([".rustfmt.toml", "rustfmt.toml"]) under path only. Bails out if none is found.
• TOML mode: If path exists and the metadata says it's not a dir, then interpret it as a TOML file. Bails out if none is found.

Proposed solution

This PR changes it to:

• Base dir mode: If path terminates with /, or if it exists and metadata says it's a dir, then search for predefined names ([".rustfmt.toml", "rustfmt.toml"]) under path and all its parents. Bails out if none is found.
• TOML mode: Otherwise, interpret it as a TOML file by searching for its filename under its parent dir and all parents of the latter. Bails out if none is found.

[rami3l]

I still some concerns regarding the distinction between the dir mode and the TOML mode, so reconverting to draft.

[rami3l]

Addressed the concern regarding mode switching; updated both the PR description accordingly to reflect the new design. This PR is ready for review 🙏