explain command (#7957)

# Description

The purpose of this PR is to introduce the `inspect` command. A command
that is used to inspect, but not run, a pipeline to ensure everything
looks right. This is meant as a debugging tool. This is some hackery, so
don't laugh. :)

![image](https://user-images.githubusercontent.com/343840/217896776-99c6bece-172c-4d3d-8cec-dda85d37cada.png)

crates/nu-command/src/system/mod.rs

@@ -2,6 +2,7 @@ mod benchmark;
 mod complete;
 #[cfg(unix)]
 mod exec;
+mod explain;
 mod nu_check;
 #[cfg(any(
     target_os = "android",
@@ -20,6 +21,7 @@ pub use benchmark::Benchmark;
 pub use complete::Complete;
 #[cfg(unix)]
 pub use exec::Exec;
+pub use explain::Explain;
 pub use nu_check::NuCheck;
 #[cfg(any(
     target_os = "android",