cratification: part III of the math commands to nu-cmd-extra (#9674)

The following math commands are being moved to nu-cmd-extra

* e (euler)
* exp
* ln

This should conclude moving the extra math commands as discussed in
yesterday's
core team meeting...

The remaining math commands will stay in nu-command (for now)....

crates/nu-cmd-extra/src/example_test.rs

@@ -15,6 +15,7 @@ mod test_examples {
         check_example_input_and_output_types_match_command_signature,
     };
 
+    use crate::MathEuler;
     use crate::MathPi;
     use nu_protocol::{
         engine::{Command, EngineState, StateWorkingSet},
@@ -65,6 +66,7 @@ mod test_examples {
             working_set.add_decl(Box::new(nu_command::Enumerate));
             working_set.add_decl(Box::new(nu_cmd_lang::If));
             // math commands
+            working_set.add_decl(Box::new(MathEuler));
             working_set.add_decl(Box::new(MathPi));
             working_set.add_decl(Box::new(nu_command::MathRound));