Implement push command

HELP.md

@@ -314,6 +314,10 @@ Example: Say `$a` is `[1, 2, 3]` and subject is `[a, b, c]` -> `zip $a` -> `[a,
 Collapse a nested destructured string down a single level, optionally limiting to a specific `index`.
 Example: `[[a,b], c, [d,e]]` -> `flatten 2` -> `[[a,b], c, d, e]`
 
+#### `push <value>`
+Append the given `value` to the current destructured.
+Example: `[foo, bar]` -> `push foo` -> `[foo, bar, foo]`
+
 
 ### Working with Variables