[Feature] support error bar in cartesian chart

[da730]

What problem does this feature solve?

extends the spec of cartesian charts like scatter plot, bar chart, line chart to support error bar. reference: https://plotly.com/python/error-bars/ This is an extension of the normal chart in Python chart. Error bars are added to the data points of the bar chart and line chart to indicate errors in various directions. It is very commonly used in the field of statistics.

What does the proposed API look like?

data=[ x: 'some x value', y: 100, error:50 ]

const spec={ type: 'line', xField: x, yField: y, errorBar:[ //more than one error bar can be added to the chart { orient: vertical or horizontal field: error //field in data representing the error （usually the standard deviation） } ] ...

}

[da730]

antv这个没看懂：https://antv-2018.alipay.com/zh-cn/g2/3.x/demo/other/column-error.html