Add exec command for Windows (#11001)

# Description
Based of the work and discussion in #10844, this PR adds the `exec`
command for Windows. This is done by simply spawning a
`std::process::Command` and then immediately exiting via
`std::process::exit` once the child process is finished. The child
process's exit code is passed to `exit`.

# User-Facing Changes
The `exec` command is now available on Windows, and there should be no
change in behaviour for Unix systems.

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

@@ -1,5 +1,4 @@
 mod complete;
-#[cfg(unix)]
 mod exec;
 mod nu_check;
 #[cfg(any(
@@ -16,7 +15,6 @@ mod sys;
 mod which_;
 
 pub use complete::Complete;
-#[cfg(unix)]
 pub use exec::Exec;
 pub use nu_check::NuCheck;
 #[cfg(any(