`devcontainer` tool mingles stdout and stderr

I'm attempting to use `devcontainer` but it is difficult because even in JSON mode, the tool prints Node warnings to stdout:

```
$ devcontainer --log-format=json read-configuration | jq
parse error: Invalid numeric literal at line 1, column 6
```

Looking at the stdout, it's clear that non-JSON content is in the output:
```
$ devcontainer --log-format=json read-configuration | grep -v '^{'
(node:847) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
(Use `Code Helper --trace-deprecation ...` to show where the warning was created)
```

this makes using `devcontainer` from a program more difficult than necessary.