[spec] Allow trailing comma in array

[at15]

In format like JSON, trailing comma is not allowed [1, 2, 3,] however, when write in multiple lines, trailing comma is really helpful for comment in/out a single line, and make copy and paste easier

backends = [
    "abc",
    // "def",
    "ghi", 
]

TODO

• adjust the grammar file, was using [value (, value)*], need to see if this would cause ambiguous in parser ...
• add test in other language implementation

Ref

• https://github.com/apple/swift-evolution/blob/master/proposals/0084-trailing-commas.md
• http://docs.scala-lang.org/sips/completed/trailing-commas.html