Make the math commands const (#13566)

This PR closes [Issue
#13482](https://github.com/nushell/nushell/issues/13482)

# Description
This PR tend to make all math function to be constant. 

# User-Facing Changes
The math commands now can be used as constant methods.

### Some Example
```
> const MODE = [3 3 9 12 12 15] | math mode
> $MODE
╭───┬────╮
│ 0 │  3 │
│ 1 │ 12 │
╰───┴────╯

> const LOG = [16 8 4] | math log 2
> $LOG
╭───┬──────╮
│ 0 │ 4.00 │
│ 1 │ 3.00 │
│ 2 │ 2.00 │
╰───┴──────╯

> const VAR = [1 3 5] | math variance
> $VAR
2.6666666666666665
```

# Tests + Formatting
Tests are added for all of the math command to test there constant
behavior.

I mostly focused on the actual user experience, not the correctness of
the methods and algorithms.

# After Submitting
I think this change don't require any additional documentation. Feel
free to correct me in this topic please.

crates/nu-command/tests/commands/math/round.rs

@@ -34,3 +34,9 @@ fn fails_with_wrong_input_type() {
 
     assert!(actual.err.contains("command doesn't support"))
 }
+
+#[test]
+fn const_round() {
+    let actual = nu!("const ROUND = 18.345 | math round; $ROUND");
+    assert_eq!(actual.out, "18");
+}