Add engines that have implemented the proposal

[linusg]

There are now at least two implementations of the Object.hasOwn() proposal:

• SerenityOS's LibJS: https://github.com/SerenityOS/serenity/commit/3ee092cd0cacb999469e50aa5ff220e397df2d79 - 2021-05-18
• SpiderMonkey: https://hg.mozilla.org/try/rev/94515f78324e83d4fd84f4b0ab764b34aabe6d80 - 2021-05-19

[ljharb]

SpiderMonkey’s (correctly) hasn’t landed yet, since the proposal isn’t stage 3.

[jamiebuilds]

Now that this proposal has reached stage 3 I've updated the implementation list to note the in progress work.