add term size command (#792)

* add `term-size` command

* Update term_size.rs

Co-authored-by: JT <547158+jntrnr@users.noreply.github.com>

crates/nu-command/src/default_context.rs

@@ -172,6 +172,7 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             Input,
             Kill,
             Sleep,
+            TermSize,
         };
 
         // Date