Implement config subcommand

The synopsis is as follows.

List all top-level config keys:

    $ ruff config
    allowed-confusables
    builtins
    cache-dir
    ... etc.

List all config keys in a specific section:

    $ ruff config mccabe
    max-complexity

Describe a specific config option:

    $ ruff config mccabe.max-complexity
    The maximum McCabe complexity to allow before triggering `C901` errors.

    Default value: 10
    Type: int
    Example usage:
    ```toml
    # Flag errors (`C901`) whenever the complexity level exceeds 5.
    max-complexity = 5
    ```

crates/ruff_cli/src/commands/mod.rs

@@ -9,6 +9,7 @@ pub use show_settings::show_settings;
 
 mod add_noqa;
 mod clean;
+pub mod config;
 mod linter;
 mod rule;
 mod run;