Advance to stage 4

[mathiasbynens]

Criteria taken from the TC39 process document minus those from previous stages:

• Test262 acceptance tests have been written for mainline usage scenarios, and merged

PR by @gibson042: tc39/test262#1787

• Two compatible implementations which pass the acceptance tests

https://github.com/tc39/proposal-well-formed-stringify#implementations

Bug tickets to track:

• Chrome/V8: https://bugs.chromium.org/p/v8/issues/detail?id=7782
• Edge/Chakra: Implement well-formed JSON.stringify proposal chakra-core/ChakraCore#5735
• Firefox/SpiderMonkey: https://bugzilla.mozilla.org/show_bug.cgi?id=1469021
• Safari/JavaScriptCore: https://bugs.webkit.org/show_bug.cgi?id=190110

• Significant in-the-field experience with shipping implementations, such as that provided by two independent VMs

https://github.com/tc39/proposal-well-formed-stringify#implementations

• A pull request has been sent to https://github.com/tc39/ecma262 with the integrated spec text

PR by @gibson042: tc39/ecma262#1396

• The ECMAScript editor has signed off on the pull request

TODO

[mathiasbynens]

Note that the spec PR, when created, can reference "Closes tc39/ecma262#944" in its commit message.