feat: add underlying Command accessors to *CommandWrap

[9999years]

Port of watchexec/command-group#27.

Hello! I'm implementing command-error, a crate that provides nicer error messages for commands, like this:

`sh` failed: exit status: 1
Command failed: `sh -c 'echo puppy; false'`
Stdout:
  puppy

I'd like to add optional support for the process-wrap crate (9999years/command-error#2). Because traits can only be implemented once per type (and I think a generic trait would have poor ergonomics here), I'm attempting to impl command_error::CommandExt for process_wrap::std::StdCommandWrap, but it's not possible to access the underlying Command (to produce the name of the program that was run for error messages) with the current API.

Therefore, this PR adds getters to access the Command field of StdCommandWrap.

[9999years]

Thanks! Let me know when there's a release cut for this?