Attempts to add // math operator (#5759)

* attempts to add `div` math operator

* allows `//` to be used too

* fmt:

* clippy issue

* returns appropriate type

* returns appropriate type 2

* fmt

* ensure consistency; rename to `fdiv`

* Update parser.rs

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

@@ -30,7 +30,10 @@ impl Expression {
 
                 match operator {
                     Operator::Pow => 100,
-                    Operator::Multiply | Operator::Divide | Operator::Modulo => 95,
+                    Operator::Multiply
+                    | Operator::Divide
+                    | Operator::Modulo
+                    | Operator::FloorDivision => 95,
                     Operator::Plus | Operator::Minus => 90,
                     Operator::NotRegexMatch
                     | Operator::RegexMatch