unify the *-BuiltinVar parser errors (#8944)

# Description

this pr condenses `MutBuiltinVar`, `LetBuiltinVar` and `ConstBuiltinVar`
into one error:
```nu
Error: nu::parser::name_is_builtin_var

  × `in` used as variable name.
   ╭─[entry #69:1:1]
 1 │ let in = 420
   ·     ─┬
   ·      ╰── already a builtin variable
   ╰────
  help: 'in' is the name of a builtin Nushell variable and cannot be used
        as a variable name
```

it also fixes this case which was previously not handled
```nu
let $nu = 420 # this variable would have been 'lost'
```

crates/nu-command/tests/commands/let_.rs

@@ -1,7 +1,7 @@
 use nu_test_support::{nu, pipeline};
 
 #[test]
-fn let_parse_error() {
+fn let_name_builtin_var() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"

crates/nu-command/tests/commands/mut_.rs

@@ -12,6 +12,34 @@ fn mut_variable() {
     assert_eq!(actual.out, "4");
 }
 
+#[test]
+fn mut_name_builtin_var() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut in = 3
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("'in' is the name of a builtin Nushell variable"));
+}
+
+#[test]
+fn mut_name_builtin_var_with_dollar() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut $env = 3
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("'env' is the name of a builtin Nushell variable"))
+}
+
 #[test]
 fn mut_variable_in_loop() {
     let actual = nu!(

crates/nu-parser/src/parse_keywords.rs

@@ -2439,15 +2439,11 @@ pub fn parse_let_or_const(working_set: &mut StateWorkingSet, spans: &[Span]) ->
 
                         let var_name =
                             String::from_utf8_lossy(working_set.get_span_contents(lvalue.span))
+                                .trim_start_matches('$')
                                 .to_string();
 
                         if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
-                            if is_const {
-                                working_set
-                                    .error(ParseError::ConstBuiltinVar(var_name, lvalue.span))
-                            } else {
-                                working_set.error(ParseError::LetBuiltinVar(var_name, lvalue.span))
-                            };
+                            working_set.error(ParseError::NameIsBuiltinVar(var_name, lvalue.span))
                         }
 
                         let var_id = lvalue.as_var();
@@ -2551,10 +2547,11 @@ pub fn parse_mut(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline
 
                         let var_name =
                             String::from_utf8_lossy(working_set.get_span_contents(lvalue.span))
+                                .trim_start_matches('$')
                                 .to_string();
 
                         if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
-                            working_set.error(ParseError::MutBuiltinVar(var_name, lvalue.span));
+                            working_set.error(ParseError::NameIsBuiltinVar(var_name, lvalue.span))
                         }
 
                         let var_id = lvalue.as_var();

crates/nu-protocol/src/parse_error.rs

@@ -142,26 +142,14 @@ pub enum ParseError {
     )]
     AssignInPipeline(String, String, String, #[label("'{0}' in pipeline")] Span),
 
-    #[error("Let used with builtin variable name.")]
+    #[error("`{0}` used as variable name.")]
     #[diagnostic(
-        code(nu::parser::let_builtin_var),
-        help("'{0}' is the name of a builtin Nushell variable. `let` cannot assign to it.")
+        code(nu::parser::name_is_builtin_var),
+        help(
+            "'{0}' is the name of a builtin Nushell variable and cannot be used as a variable name"
+        )
     )]
-    LetBuiltinVar(String, #[label("already a builtin variable")] Span),
-
-    #[error("Const used with builtin variable name.")]
-    #[diagnostic(
-        code(nu::parser::let_builtin_var),
-        help("'{0}' is the name of a builtin Nushell variable. `const` cannot assign to it.")
-    )]
-    ConstBuiltinVar(String, #[label("already a builtin variable")] Span),
-
-    #[error("Mut used with builtin variable name.")]
-    #[diagnostic(
-        code(nu::parser::let_builtin_var),
-        help("'{0}' is the name of a builtin Nushell variable. `mut` cannot assign to it.")
-    )]
-    MutBuiltinVar(String, #[label("already a builtin variable")] Span),
+    NameIsBuiltinVar(String, #[label("already a builtin variable")] Span),
 
     #[error("Incorrect value")]
     #[diagnostic(code(nu::parser::incorrect_value), help("{2}"))]
@@ -445,9 +433,7 @@ impl ParseError {
             ParseError::CantAliasExpression(_, s) => *s,
             ParseError::BuiltinCommandInPipeline(_, s) => *s,
             ParseError::AssignInPipeline(_, _, _, s) => *s,
-            ParseError::LetBuiltinVar(_, s) => *s,
-            ParseError::MutBuiltinVar(_, s) => *s,
-            ParseError::ConstBuiltinVar(_, s) => *s,
+            ParseError::NameIsBuiltinVar(_, s) => *s,
             ParseError::CaptureOfMutableVar(s) => *s,
             ParseError::IncorrectValue(_, s, _) => *s,
             ParseError::MultipleRestParams(s) => *s,