Allow config to work with column paths. (#2653)

Nu has many commands that allow the nuño to customize behavior such
as UI and behavior. Today, coloring can be customized, the line editor,
and other things. The more options there are, the higher the complexity
in managing them.

To mitigate this Nu can store configuration options as nested properties.

But to add and edit them can be taxing. With column path support we can
work with them easier.

crates/nu-protocol/src/value.rs

@@ -92,6 +92,11 @@ impl UntaggedValue {
         matches!(self, UntaggedValue::Table(_))
     }
 
+    /// Returns true if this value represents a row
+    pub fn is_row(&self) -> bool {
+        matches!(self, UntaggedValue::Row(_))
+    }
+
     /// Returns true if this value represents a string
     pub fn is_string(&self) -> bool {
         matches!(self, UntaggedValue::Primitive(Primitive::String(_)))