Support JSON6 (#10456)
* Add test
* Support JSON6
* Test
* Print `null` instead of holes
* More tests
* Update changelog
* Style
* Minor refactor
* Fix AST clean
* Fix `Identifier` clean
* Fix AST compare
* Style
* Reword changelog
Co-authored-by: Georgii Dolzhykov <thorn.mailbox@gmail.com>
