Option to replace command same name (#374)

* option to replace command same name

* moved order of custom value declarations

* arranged dataframe folders and objects

* sort help commands by name

* added dtypes function for debugging

* corrected name for dataframe commands

* command names using function

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

@@ -1,12 +1,14 @@
 use std::fmt;
 
-use crate::{ast::Operator, ShellError, Span, Value};
+use crate::{ast::Operator, Category, ShellError, Span, Value};
 
 // Trait definition for a custom value
 #[typetag::serde(tag = "type")]
 pub trait CustomValue: fmt::Debug + Send + Sync {
     fn clone_value(&self, span: Span) -> Value;
 
+    fn category(&self) -> Category;
+
     // Define string representation of the custom value
     fn value_string(&self) -> String;