[Feature] Configure/disable label text border and drop shadow

### What problem does this feature solve?

If you look at the theme builder on a dark theme, you will see that all labels have a border and a drop-shadow (I believe):

I discussed this on [Stack Overflow](https://stackoverflow.com/questions/76465733/how-to-remove-white-outline-from-label-text-in-apache-e-charts-bar-chart/76466735?noredirect=1#comment136897241_76466735) and was encouraged to raise an issue.

As I understand the answer on SO, there are several cases:
* the text is over a plot
* the text is not over a plot
* the text is halfway over a plot

My personal impression is that borders/drop-shadow on text never really improves readability. So I'd rather completely disable it and possibly provide a uniform background where there is an overlap.

### What does the proposed API look like?

I'd rather leave it to someone more experimented on echarts than I am to figure out the API.