js: add duktape, support both mujs and duktape.

This PR is only to allow inline comments on the overall duktape support commits.
I think the behavior is defined at sections 6.10.3.2 and 6.10.3.3 at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf .

From only skimming the text, nothing obvious popped to my eyes that says it shouldn't work, but it melted my brain and I can't say I fully followed all the cases and exceptions.