Add ppid to ps command (#8750)

# Description

Adds the `ppid` field that's available on all supported platforms to the
`ps` command. This would be useful in my scripts.

# User-Facing Changes

- ps output now contains an extra column

# Tests + Formatting

Not sure if I need to add a test for this

# After Submitting

Update https://www.nushell.sh/book/quick_tour.html#quick-tour to show
the new table

crates/nu-system/src/linux.rs

@@ -131,6 +131,11 @@ impl ProcessInfo {
         self.pid
     }
 
+    /// PPID of process
+    pub fn ppid(&self) -> i32 {
+        self.ppid
+    }
+
     /// Name of command
     pub fn name(&self) -> String {
         self.command()

crates/nu-system/src/macos.rs

@@ -302,6 +302,11 @@ impl ProcessInfo {
         self.pid
     }
 
+    /// Parent PID of process
+    pub fn ppid(&self) -> i32 {
+        self.ppid
+    }
+
     /// Name of command
     pub fn name(&self) -> String {
         if let Some(path) = &self.curr_path {

crates/nu-system/src/windows.rs

@@ -1006,6 +1006,11 @@ impl ProcessInfo {
         self.pid
     }
 
+    /// Parent PID of process
+    pub fn ppid(&self) -> i32 {
+        self.ppid
+    }
+
     /// Name of command
     pub fn name(&self) -> String {
         self.command.clone()