prevent parser from parsing variables as units (#12378)

# Description

Resolves #11274.

```
~/CodingProjects/nushell> let day = 2; echo 0..<$day
╭───┬───╮
│ 0 │ 0 │
│ 1 │ 1 │
╰───┴───╯
~/CodingProjects/nushell> let kb = "jan"; echo 0..$kb 
Error: nu:🐚:type_mismatch

  × Type mismatch during operation.
   ╭─[entry #1:1:22]
 1 │ let kb = "jan"; echo 0..$kb
   ·                      ┬─┬─┬─
   ·                      │ │ ╰── string
   ·                      │ ╰── type mismatch for operator
   ·                      ╰── int
   ╰────
```


# Tests + Formatting

Relevant test added 🆙 

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-parser/tests/test_parser.rs

@@ -1191,6 +1191,16 @@ mod range {
 
         assert!(!working_set.parse_errors.is_empty());
     }
+
+    #[test]
+    fn vars_not_read_as_units() {
+        let engine_state = EngineState::new();
+        let mut working_set = StateWorkingSet::new(&engine_state);
+
+        let _ = parse(&mut working_set, None, b"0..<$day", true);
+
+        assert!(working_set.parse_errors.is_empty());
+    }
 }
 
 #[cfg(test)]