Simplify stacking even more, leverage more ES6 features

[kurkle]

We don't actually support a case where both scales values are stacked. Its always the value scale. Index scale can have stack groups, but its controlled by stack option of dataset and appended to the stack key.

Destructuring assignment helps reduce lines.
Computed property names in object initializer reduce the need for temporary variables.

parsed.push({
	[xId]: xScale._parseObject(item, 'x', i),
	[yId]: yScale._parseObject(item, 'y', i),
});

translates to:

parsed.push((_parsed$push4 = {}, _defineProperty(_parsed$push4, xId, xScale._parseObject(item, 'x', i)), _defineProperty(_parsed$push4, yId, yScale._parseObject(item, 'y', i)), _parsed$push4));

And that is faster (in Chrome)! (8.2 ms vs 12.4 ms, uPlot)

Force push: removed unintended inclusion of #6768

[benmccann]

@kurkle this PR will need to be rebased