taplo-cli: add ENV variable support

[razvanazamfirei]

This pull request improves the configuration file lookup logic in taplo-cli. The changes enable the CLI to search for a configuration file in the TAPLO_CONFIG environment variable if no configuration file is found in the current working directory and no explicit path has been provided by the user.

Changes:

1. Added support for TAPLO_CONFIG in the load_config function.
2. Minor documentation updates

[ia0]

Thanks for the PR, that's a good idea!

[razvanazamfirei]

Thanks, @ia0! Changes made.

[ia0]

Thanks!

You also need to add the env feature to clap.

-clap = { version = "3.0.0", features = ["derive", "cargo"] }
+clap = { version = "3.0.0", features = ["derive", "cargo", "env"] }

[ia0]

Thanks!