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>

changelog_unreleased/json/10346.md

@@ -1,6 +1,6 @@
-#### Stricter JSON parser (#10346, #10443 by @fisker)
+#### Stricter JSON parser (#10346, #10443, #10456 by @fisker)
 
-Previously we allow all kinds of JavaScript expressions in `json` and `json5` parser, now they are stricter.
+Prettier internally uses a JavaScript expression parser to parse JSON. That's why the `json` and `json5` parsers used to be very forgiving and allowed all kinds of JavaScript expressions. Now they are much stricter, although some simple non-standard syntax is still allowed (e.g., [JSON6](https://github.com/d3x0r/JSON6) is supported, except for multiple minus signs: `----123`).
 
 <!-- prettier-ignore -->
 ```js