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!(