Errors when let in, let env and similar commands are passed. (#5866)

* throw `let nu/env/nothing/in` error in parsing

* add tests and fmt

* fix clippy

* suggestions

* fmt

* `lvalue.span` instead of `spans[1]`

* clippy

* fmt

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

@@ -0,0 +1,15 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn let_parse_error() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        let in = 3
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("'in' is the name of a builtin Nushell variable"));
+}

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

@@ -30,6 +30,7 @@ mod into_filesize;
 mod into_int;
 mod last;
 mod length;
+mod let_;
 mod lines;
 mod ls;
 mod math;