bpo-36142: Remove _PyMain structure (GH-12120)

* Move fields from _PyMain to _PyCoreConfig:

  * skip_first_line
  * run_command
  * run_module
  * run_filename

* Replace _PyMain.stdin_is_interactive with a new
  stdin_is_interactive(config) function
* Rename _PyMain to _PyArgv. Add "const _PyArgv *args" field
  to _PyCmdline.