[jsinterp] Improve JS language support (#31175)

* operator ??
* operator ?.
* operator **
* accurate operator functions
* `undefined` handling
* object literals {a: 1, "b": expr}
* more tests for weird JS comparisons: see https://github.com/ytdl-org/youtube-dl/issues/31173#issuecomment-1217854397.

test/test_youtube_signature.py

@@ -102,6 +102,10 @@ _NSIG_TESTS = [
         'https://www.youtube.com/s/player/4c3f79c5/player_ias.vflset/en_US/base.js',
         'TDCstCG66tEAO5pR9o', 'dbxNtZ14c-yWyw',
     ),
+    (
+        'https://www.youtube.com/s/player/c81bbb4a/player_ias.vflset/en_US/base.js',
+        'gre3EcLurNY2vqp94', 'Z9DfGxWP115WTg',
+    ),
 ]