[Feature] Apply styling on a stack level for bar charts

### What problem does this feature solve?

Currently, styling is applied to each individual series/data point, but there's no way to apply it to the whole stacked group. Therefore, some styling may look strange since the bar isn't considered as a whole when applying the styling.

2 concrete examples where this issue matters are borders (which cut into the bar) and rounded corners (which get repeated in the middle of the graph).

This is how a dashed border looks on a stacked bar currently:

And this is how it should look:

This is how rounded top corners look on a stacked bar currently:

And this is how they should look:

### What does the proposed API look like?

The simplest I can think of is to introduce a new optional field in `BarItemStyleOption`, which could change where the styling is applied, for example `target?: 'stack' | 'series'`.

Start by locating BarItemStyleOption and the stacked bar rendering entry points, then trace how borders and rounded corners are currently applied to individual series or data points. Done should provide an agreed API for stack-level targeting and make dashed borders and rounded top corners render once around the stacked group.

Written by the indexing model from the issue text.