feat(output): redirect trace output to stderr

[thevilledev]

This change standardizes the behavior of the --trace flag. When --trace is used, trace output is now consistently directed to stderr, while the primary formatted output (controlled by --output) remains on stdout.

Previously, combining --trace with non-standard output formats (e.g., --output=json, --output=table) resulted in trace output being suppressed entirely. This update resolves that limitation, enabling users to capture detailed trace information separately from the main results, regardless of the chosen output format. For instance, trace logs can be saved to a file using stderr redirection:

conftest test --trace --output=table my_config.yaml 2> trace.log

BREAKING CHANGE: Trace output is now always sent to stderr when --trace is enabled, including when using the default (stdout) output format. Previously, trace output for the default format went to stdout. Users with scripts or workflows that captured trace logs from stdout will need to update them to read from stderr instead.

Fixes #1031

[jalseth]

Thanks for taking this on. I am thinking we should just use stderr for all outputters, including the standard one. I am OK with this being a (very unlikely) breaking change as nobody should rely on trace output for anything other than debugging.

[thevilledev]

Sounds good to me! Updated the PR to use stdout/stderr the same way for all output formats.