Node.js 12.4.0 release, JSON document stable

Significant changes

• doc：
  • JSON API documentation variants is no longer experimental (Rich Trott) # 27 842 .
• Esm :
  • JSON module support is always enabled --experimental-modules. The
    --experimental-json-modulesbanner has been deleted (Myles Borins) # 27752 .
• http，http2：
  • Added a new flag to override the default HTTP server socket
    timeout (two minutes). By --http-server-default-timeout=milliseconds
    or --http-server-default-timeout=0separately to change or disable the timeout.
    Starting Node.js 13.0.0, by default disables the timeout (Ali Ijaz Sheikh) # 27704 .
• inspector：
  • Added an experimental --heap-profflag to
    start the V8 heap analyzer at startup, and before exiting the stack configuration file is written to disk (Joyee Cheung,) # 27596 .
• Flow :
  • This readable.unshift()method is now correctly convert a string buffer 
    addition, to accept a new optional parameters specified string
    encoding, for example, 'utf8'or 'ascii'(Marcos Casagrande) # 27194 .
• v8 :
  • The returned object v8.getHeapStatistics()has two new properties:
    number_of_native_contextsand number_of_detached_contexts(Yuriy Vasiyarov) # 27933 .