Support creating YAML config files via `devproxy config new`

[garrytrinder]

Summary

devproxy config new currently defaults to creating a devproxyrc.json file and only generates JSON configs. With YAML config support being added (#1491), the CLI should also support creating YAML-based configuration files.

Expected behavior

• devproxy config new devproxyrc.yaml should create a valid YAML config file
• The format should be inferred from the file extension (.yaml/.yml = YAML, .json/.jsonc = JSON)
• The default should remain devproxyrc.json for backward compatibility

Current behavior

devproxy config new only generates JSON configs regardless of the filename extension. It downloads a JSON snippet from the snippets URL and writes it as-is.

Additional context

Discovered while testing PR #1491 (YAML config support).