New termsize command (#3038)

* add term size command

* update w & h, add examples

* changed default to output table

crates/nu-command/src/commands/default_context.rs

@@ -237,6 +237,7 @@ pub fn create_default_context(interactive: bool) -> Result<EvaluationContext, Bo
             whole_stream_command(UrlQuery),
             whole_stream_command(Seq),
             whole_stream_command(SeqDates),
+            whole_stream_command(TermSize),
         ]);
 
         #[cfg(feature = "clipboard-cli")]