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-parser/src/parser.rs

@@ -4086,6 +4086,7 @@ pub fn parse_operator(
         b"-" => Operator::Minus,
         b"*" => Operator::Multiply,
         b"/" => Operator::Divide,
+        b"//" => Operator::FloorDivision,
         b"in" => Operator::In,
         b"not-in" => Operator::NotIn,
         b"mod" => Operator::Modulo,