Add parser improvements (#2679)

* Add parser improvements

Previously everything starting with "$" was parsed as a column path.
With this commit applied, the lite_arg starting with $ is parsed as
the most appropriate thing
- $true/$false ==> Expression::Boolean
- $(...) ==> Invocation
- $it ==> ColumnPath
- Anything with at least one '.' ==> ColumnPath
- Anything else ==> Variable

* Ignore failing tests

crates/nu-protocol/src/hir.rs

@@ -1154,6 +1154,10 @@ impl Expression {
             Expression::Variable(Variable::Other(v, span))
         }
     }
+
+    pub fn boolean(b: bool) -> Expression {
+        Expression::Boolean(b)
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Serialize, Deserialize)]