图例和图表之间的间距过大

- [ ] I have searched the [issues](https://github.com/antvis/f2/issues) of this repository and believe that this is not a duplicate.

### What problem does this feature solve?
图例和图表之间的间距过大，且不能设置，希望可以设置

### What does the proposed API look like?
chart.legend({
position: 'top',
top: -10
});

Start with the chart.legend API and the code that positions legends relative to the chart; no file or test is named in the issue. Confirm how the proposed top spacing option should be interpreted, then add coverage showing that the configured spacing is applied when the legend is positioned above the chart.

Written by the indexing model from the issue text.