Allow 'trailing commas' or ellipsis while defining array, map and set literals

Allowing trailing commas in structured literals (array, set, map) as a convenient syntactic leniency.

let m = { "foo": 1, "bar": 2, };
let a = [ 1, 2, ];
let s = (1, 2, );

Using a trailing ellipsis (...) to alter the concrete class materialized as a convenient semantic shortcut.

let m = { "foo": 1, "bar": 2, ... }; // LinkedHashMap
let a = [ 1, 2, ...]; // ArrayList
let s = (1, 2, ...); // LinkedHashSet