add dedicated const in pipeline, const builtin var errors (#7784)

# Description

Currently `let` and `const` share error handling, and this might lead to
confusing error messages


![sJan17-51](https://user-images.githubusercontent.com/98623181/212981108-c80b3e55-cece-4ee5-ba8f-cb5dcfdf63e0.png)

This PR adds dedicated errors to `const`

crates/nu-parser/src/parser.rs

@@ -5001,7 +5001,7 @@ pub fn parse_expression(
                 .0,
                 Some(ParseError::BuiltinCommandInPipeline("for".into(), spans[0])),
             ),
-            b"let" | b"const" => (
+            b"let" => (
                 parse_call(
                     working_set,
                     &spans[pos..],
@@ -5024,6 +5024,29 @@ pub fn parse_expression(
                     spans[0],
                 )),
             ),
+            b"const" => (
+                parse_call(
+                    working_set,
+                    &spans[pos..],
+                    spans[0],
+                    expand_aliases_denylist,
+                    is_subexpression,
+                )
+                .0,
+                Some(ParseError::ConstInPipeline(
+                    String::from_utf8_lossy(match spans.len() {
+                        1 | 2 | 3 => b"value",
+                        _ => working_set.get_span_contents(spans[3]),
+                    })
+                    .to_string(),
+                    String::from_utf8_lossy(match spans.len() {
+                        1 => b"variable",
+                        _ => working_set.get_span_contents(spans[1]),
+                    })
+                    .to_string(),
+                    spans[0],
+                )),
+            ),
             b"mut" => (
                 parse_call(
                     working_set,