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/tests/commands/mod.rs

@@ -24,7 +24,6 @@ mod echo;
 mod empty;
 mod error_make;
 mod every;
-#[cfg(not(windows))]
 mod exec;
 mod export_def;
 mod fill;