Add logical xor operator (#7242)

We already have the binary `bit-xor` and the shortcircuiting logical
`or`(`||`) and `and`(`&&`).
This introduces `xor` as a compact form for both brevity and clarity.
You can express the operation through `not`/`and`/`or` with a slight
risk of introducing bugs through typos.

Operator precedence

`and` > `xor` > `or`

Added logic and precedence tests.

crates/nu-engine/src/eval.rs

@@ -404,6 +404,10 @@ pub fn eval_expression(
                                 lhs.or(op_span, &rhs, expr.span)
                             }
                         }
+                        Boolean::Xor => {
+                            let rhs = eval_expression(engine_state, stack, rhs)?;
+                            lhs.xor(op_span, &rhs, expr.span)
+                        }
                     }
                 }
                 Operator::Math(math) => {