Implements Array.prototype.fill.

This implementation slightly differs from the ECMAScript specification in that it will extend the array until the given length (if given), rather than only filling until the original length and stopping, as is specified by the ECMAScript specification.

This alternate behavior is justified by the fact that there is not really any other convenient way of initializing an array of specific length. (new Array() constructor doesn't work, Lua arrays are just hash tables anyway). So with this slight modification you can initialize an array of given length as [].fill(defaultValue, 0, desiredLength) (might need some casting if you use an empty literal).