Add math pi and math e constants (#7258)

Currently implemented as commands

Work towards #7073

Add them to the example test harness together with `math round`

crates/nu-command/src/default_context.rs

@@ -405,6 +405,8 @@ pub fn create_default_context() -> EngineState {
             MathStddev,
             MathSum,
             MathVariance,
+            MathPi,
+            MathEuler,
         };
 
         // Network