Disallow invalid escape sequences in JSON.parse

[RedMser]

Part of #66114

According to the JSON specs, following is the definition of a character:

char
    any-Unicode-character-
        except-"-or-\-or-
        control-character
    \"
    \\
    \/
    \b
    \f
    \n
    \r
    \t
    \u four-hex-digits 

So any unlisted backslash escape will now cause a parsing error, instead of being silently accepted.

Add unit tests to ensure this behavior.

[akien-mga]

Sorry for the delay reviewing.

Seems fine to me, though this probably needs testing to ensure that it won't break user expectations somewhat. A bit late for this in the 4.1 release cycle so let's merge for 4.2 early on.

Could be good to add more unit tests to validate this behavior.

[akien-mga]

Looks great!

[YuriSizov]

Thanks!