Add configurations for table headers (#1537)

* Add configurations for table headers

* lint

Co-authored-by: Amanita-Muscaria <nope>

crates/nu-protocol/src/value/primitive.rs

@@ -74,6 +74,16 @@ impl Primitive {
             )),
         }
     }
+
+    pub fn into_string(self, span: Span) -> Result<String, ShellError> {
+        match self {
+            Primitive::String(s) => Ok(s),
+            other => Err(ShellError::type_error(
+                "string",
+                other.type_name().spanned(span),
+            )),
+        }
+    }
 }
 
 impl num_traits::Zero for Primitive {