Add support for function-comma-newline-after

[keithjgrant]

https://stylelint.io/user-guide/rules/function-comma-newline-after/

When you have a really long function, like the one below, stylefmt smashes it all onto the same line. It would be nice to have an option to keep them separate:

  background-image: linear-gradient(
    to bottom,
    hsla(200, 50%, 10%, 0.3),
    hsla(300, 50%, 10%, 0.3),
    hsla(100, 50%, 10%, 0.3),
    hsla(200, 50%, 10%, 0.3),
    hsla(300, 50%, 10%, 0.3),
    hsla(200, 50%, 10%, 0.3),
  );

stylefmt currently produces:

  background-image: linear-gradient(to bottom, hsla(200, 50%, 10%, 0.3), hsla(300, 50%, 10%, 0.3), hsla(100, 50%, 10%, 0.3), hsla(200, 50%, 10%, 0.3), hsla(300, 50%, 10%, 0.3), hsla(200, 50%, 10%, 0.3),);

…which is not readable.