Add stepping to ranges & enable reverse ranges

Follows the following syntax: <start>..<next-value>..<end>

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

@@ -7,8 +7,9 @@ pub enum Expr {
     Int(i64),
     Float(f64),
     Range(
-        Option<Box<Expression>>,
-        Option<Box<Expression>>,
+        Option<Box<Expression>>, // from
+        Option<Box<Expression>>, // next value after "from"
+        Option<Box<Expression>>, // to
         RangeOperator,
     ),
     Var(VarId),

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

@@ -59,6 +59,7 @@ pub enum RangeInclusion {
 pub struct RangeOperator {
     pub inclusion: RangeInclusion,
     pub span: Span,
+    pub next_op_span: Span,
 }
 
 impl Display for RangeOperator {