Support postcss plugin array syntax

[phoenisx]

Default way of defining plugins using postcss.config file:

{
  "plugins: {
    "autoprefixer": {} // Pass Options as object property value.
  }
}

Below is a syntax that Eslint accepts in its rules property, where options are passed as second value in an array:

{
  "plugins: [
    ["tailwindcss", {}], // Array to pass options
    "autoprefixer" // Plain string works as is
  ]
}

This syntax is to support backwards compatibility. I can remove this syntax once the extension version reaches to v3.