AssertionErrors should use process.stderr instead of console.log

Would be nice if I had the option to mute AssertionError traces by piping stderr elsewhere. I recommend a new reporter, maybe "default-pretty" and "default-verbose" which produce the same results but with the assertions errors going to stderr

The issue names no file or test. Start by locating the reporter entry points and the current console.log handling for AssertionErrors, then run the existing test suite to understand output behavior. Done means the proposed reporter behavior preserves existing results while allowing assertion traces to be redirected through stderr.

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