[Feature] Support letter-spacing and word-spacing Properties in Legend Text

### What problem does this feature solve?

This allows developers to adjust the `letter-spacing` and `word-spacing` properties without the legend text overlapping with the icon. A developer might need to set these styling properties because of the designs they were given or they might need to support it for WCAG accessibility compliance (1.4.12 Text Spacing [Level AA] – WCAG 2.1).

The following screenshot shows how the legend text overlaps with the icon if you try to change the `letter-spacing` and `word-spacing` properties outside of echarts:

You can recreate the right side of the screenshot by using the [Text Spacing Editor](https://chromewebstore.google.com/detail/text-spacing-editor/amnelgbfbdlfjeaobejkfmjjnmeddaoj) chrome extension on the [stacked area chart](https://echarts.apache.org/examples/en/editor.html?c=area-stack) example page.

This could be solved by supporting the `letter-spacing` and `word-spacing` properties in the legend textStyle options.

### What does the proposed API look like?

```
setOption({
legend: {
textStyle: {
letterSpacing: 'normal', // should support em values and px values as well
wordSpacing: 'normal', // should support em values and px values as well
},
},
})
```

The legend would need to take these properties into consideration when calculating the size of each item.