Add sys users command (#12787)

# Description
Add a new `sys users` command which returns a table of the users of the
system. This is the same table that is currently present as
`(sys).host.sessions`. The same table has been removed from the recently
added `sys host` command.

# User-Facing Changes
Adds a new command. (The old `sys` command is left as is.)

crates/nu-command/src/default_context.rs

@@ -125,6 +125,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             SysMem,
             SysNet,
             SysTemp,
+            SysUsers,
             UName,
 
         };