fix: extend action should spread a scalar string by code point

lib/argparse.js

@@ -1903,7 +1903,7 @@ const _ExtendAction = _callable(class _ExtendAction extends _AppendAction {
     call (parser, namespace, values/*, option_string = undefined */) {
         let items = getattr(namespace, this.dest, undefined)
         items = _copy_items(items)
-        items = items.concat(values)
+        items = items.concat(typeof values === 'string' ? Array.from(values) : values)
         setattr(namespace, this.dest, items)
     }
 })

test/test_argparse.js

@@ -2468,6 +2468,19 @@ class WFile {
     ]
 }).run()
 
+;(new class TestActionExtendDefaultNargs extends ParserTestCase {
+    /* Without nargs, a single value is a string, and extend() spreads it
+     * character by character, same as Python's list.extend() on a str */
+
+    argument_signatures = [
+        Sig('--foo', { action: 'extend' }),
+    ]
+    failures = []
+    successes = [
+        ['--foo abc', NS({ foo: ['a', 'b', 'c'] })],
+    ]
+}).run()
+
 ;(new class TestNegativeNumber extends ParserTestCase {
     /* Test parsing negative numbers */