Allow operator in constants (#10212)

This pr fixes https://github.com/nushell/nushell/issues/10200

# Description

Allow unary and binary operators in constants, e.g.

```bash
const a = 1 + 2
const b = [0, 1, 2, 3] ++ [4]
```

# User-Facing Changes

Now constants can contain operators.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

None

---------

Co-authored-by: Horasal <horsal@horsal.dev>

crates/nu-protocol/src/ast/operator.rs

@@ -1,8 +1,10 @@
-use crate::Span;
+use crate::{ShellError, Span};
 
 use serde::{Deserialize, Serialize};
 use std::fmt::Display;
 
+use super::{Expr, Expression};
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum Comparison {
     Equal,
@@ -128,3 +130,16 @@ impl Display for RangeOperator {
         }
     }
 }
+
+pub fn eval_operator(op: &Expression) -> Result<Operator, ShellError> {
+    match op {
+        Expression {
+            expr: Expr::Operator(operator),
+            ..
+        } => Ok(operator.clone()),
+        Expression { span, expr, .. } => Err(ShellError::UnknownOperator {
+            op_token: format!("{expr:?}"),
+            span: *span,
+        }),
+    }
+}