All is a DataFrame (#3812)

* nuframe in its own type in UntaggedValue

* Removed eager dataframe from enum

* Dataframe created from list of values

* Corrected order in dataframe columns

* Returned tag from stream collection

* Removed series from dataframe commands

* Arithmetic operators

* forced push

* forced push

* Replace all command

* String commands

* appending operations with dfs

* Testing suite for dataframes

* Unit test for dataframe commands

* improved equality for dataframes

crates/nu_plugin_post/src/post.rs

@@ -406,7 +406,7 @@ pub fn value_to_json_value(v: &Value) -> Result<serde_json::Value, ShellError> {
 
         UntaggedValue::Table(l) => serde_json::Value::Array(json_list(l)?),
         #[cfg(feature = "dataframe")]
-        UntaggedValue::DataFrame(_) => {
+        UntaggedValue::DataFrame(_) | UntaggedValue::FrameStruct(_) => {
             return Err(ShellError::labeled_error(
                 "Cannot convert data struct",
                 "Cannot convert data struct",