Show std if `$XONSH_TRACE_SUBPROC=3`.
### Motivation
* It's very helpful if you want to understand how subproc is working.
* It's helpful to trace `SpecModifierAlias`.
* It's helpful to trace cases like #2618
## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**
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Co-authored-by: a <1@1.1>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
