Add math exp command (issue #8661) (#8700)

# Description
I copied the `math ln` command and replaced the relevant parts to
implement `math exp`.

# User-Facing Changes

The `math exp` command was added. Now one can do `[1, 2, 3] | math exp`
to get e to the power of these numbers.

# Tests + Formatting
I only wrote example tests, same as for `math ln`, which also does not
have special tests. I have ran into an issue with the tests but it seems
completely unrelated (see #8687)

# After Submitting

This PR was done in order to make the documentation complete, so I'm not
adding any documentation except `math ln`.

crates/nu-command/src/default_context.rs

@@ -387,6 +387,7 @@ pub fn create_default_context() -> EngineState {
             MathPi,
             MathTau,
             MathEuler,
+            MathExp,
             MathLn,
             MathLog,
         };