move math constants to standard library (nushell#9678)

# Description
we talked about this before in some meetings so i thought, why not?

the hope is that these constants do not require Rust code to be
implemented and that this move will make the Rust source base a bit
smaller 🤞

# User-Facing Changes
mathematical constants (e, pi, tau, phi and gamma) are now in `std math`
rather than `math`

## what can be done
```nushell
> use std; $std.math
> use std math; $math
> use std *; $math
```
will all give
```
╭───────┬────────────────────╮
│ GAMMA │ 0.5772156649015329 │
│ E     │ 2.718281828459045  │
│ PI    │ 3.141592653589793  │
│ TAU   │ 6.283185307179586  │
│ PHI   │ 1.618033988749895  │
╰───────┴────────────────────╯
```
and the following will work too
```nushell
> use std math E; $E
2.718281828459045
```
```nushell
> use std math *; $GAMMA
0.5772156649015329
```

## what can NOT be done
looks like every export works fine now 😌 

# Tests + Formatting
# After Submitting

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

@@ -15,8 +15,6 @@ 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},
         Type,
@@ -64,9 +62,6 @@ 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));
 
             // Adding the command that is being tested to the working set

crates/nu-cmd-extra/src/extra/math/cos.rs

@@ -54,7 +54,7 @@ impl Command for SubCommand {
         vec![
             Example {
                 description: "Apply the cosine to π",
-                example: "math pi | math cos",
+                example: "3.141592 | math cos | math round --precision 4",
                 result: Some(Value::test_float(-1f64)),
             },
             Example {

crates/nu-cmd-extra/src/extra/math/ln.rs

@@ -52,7 +52,7 @@ impl Command for SubCommand {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Get the natural logarithm of e",
-            example: "math e | math ln",
+            example: "2.7182818 | math ln | math round --precision 4",
             result: Some(Value::test_float(1.0f64)),
         }]
     }

crates/nu-cmd-extra/src/extra/math/mod.rs

@@ -5,13 +5,8 @@ mod sinh;
 mod tan;
 mod tanh;
 
-mod egamma;
-mod euler;
 mod exp;
 mod ln;
-mod phi;
-mod pi;
-mod tau;
 
 mod arccos;
 mod arccosh;
@@ -27,13 +22,8 @@ pub use sinh::SubCommand as MathSinH;
 pub use tan::SubCommand as MathTan;
 pub use tanh::SubCommand as MathTanH;
 
-pub use egamma::SubCommand as MathEulerGamma;
-pub use euler::SubCommand as MathEuler;
 pub use exp::SubCommand as MathExp;
 pub use ln::SubCommand as MathLn;
-pub use phi::SubCommand as MathPhi;
-pub use pi::SubCommand as MathPi;
-pub use tau::SubCommand as MathTau;
 
 pub use arccos::SubCommand as MathArcCos;
 pub use arccosh::SubCommand as MathArcCosH;

crates/nu-cmd-extra/src/extra/math/sin.rs

@@ -54,7 +54,7 @@ impl Command for SubCommand {
         vec![
             Example {
                 description: "Apply the sine to π/2",
-                example: "(math pi) / 2 | math sin",
+                example: "3.141592 / 2 | math sin | math round --precision 4",
                 result: Some(Value::test_float(1f64)),
             },
             Example {

crates/nu-cmd-extra/src/extra/math/tan.rs

@@ -54,7 +54,7 @@ impl Command for SubCommand {
         vec![
             Example {
                 description: "Apply the tangent to π/4",
-                example: "(math pi) / 4 | math tan",
+                example: "3.141592 / 4 | math tan | math round --precision 4",
                 result: Some(Value::test_float(1f64)),
             },
             Example {

crates/nu-cmd-extra/src/extra/math/tanh.rs

@@ -52,7 +52,7 @@ impl Command for SubCommand {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Apply the hyperbolic tangent to 10*π",
-            example: "(math pi) * 10 | math tanh",
+            example: "3.141592 * 10 | math tanh | math round --precision 4",
             result: Some(Value::test_float(1f64)),
         }]
     }