parameter options should be grouped to an array of terms

Description

I put the parameter options of superellipse() in brackets to group them into an array.

Motivation

The current version breaks parsers, for example CSSTree as used in csstype

Additional details

I followed the pattern as seen in skewX() in the same file.

Alternatively, a new "type" can be created and used like so:

  "superellipse-domain": {
    "syntax": "<number> | infinity | -infinity"
  },
  "superellipse()": {
    "syntax": "superellipse( <superellipse-domain> )"
  },

Related issues and pull requests