Add rule to warn about sparse array literals

[ajafff]

[, 1, 2]; // empty element at start
[1,, 2]; // empty element in the middle
[1, 2,,]; // empty element at the end
[1, 2,]; // OK

// no errors on destructuring assignment
[,a,,b,,c,,] = arr;

[ajafff]

For the record: those errors on destructuring assignment will be part of no-useless-destructuring proposed in #325