Remove cJSON dependency from the json module

[medvednikov]

Write a JSON parser in V.

[ntrel]

IMO it's fine to depend on C libraries for non-core functionality like JSON, networking, graphics etc. Using existing libraries helps with stability.

[nedpals]

I'll just leave a link here just in case everyone is wondering about this:
https://modules.vlang.io/x.json2.html

[nedpals]

Closed in favor of #7032 .