Make $ on variable names optional (#6434)

* DRAFT: make var dollar optional

* couple fixes

* fix some tests + cleanup

crates/nu-parser/src/parser.rs

@@ -74,6 +74,10 @@ pub fn is_math_expression_like(
         return true;
     }
 
+    if bytes == b"nu" {
+        return false;
+    }
+
     let b = bytes[0];
 
     if b == b'('
@@ -115,6 +119,11 @@ pub fn is_math_expression_like(
         return true;
     }
 
+    let parsed_variable = parse_variable(working_set, span);
+    if parsed_variable.0.is_some() && parsed_variable.1.is_none() {
+        return true;
+    }
+
     false
 }
 
@@ -4008,6 +4017,23 @@ pub fn parse_value(
         return parse_variable_expr(working_set, span);
     }
 
+    let parsed_variable = parse_variable(working_set, span);
+    if parsed_variable.0.is_some() && parsed_variable.1.is_none() {
+        let var_id = parsed_variable
+            .0
+            .expect("internal error: already checked var id exists");
+        return (
+            Expression {
+                expr: Expr::Var(var_id),
+                span,
+                custom_completion: None,
+                ty: working_set.get_variable(var_id).ty.clone(),
+            },
+            None,
+        );
+    }
+    let bytes = working_set.get_span_contents(span);
+
     // Check for reserved keyword values
     match bytes {
         b"true" => {