Component | XYLables: Setting stroke via CSS variables

[rokotyan]

Fixes #98. Stroke can now be set via CSS variables:

.stackBarLabels g[alert] {
  --vis-xy-label-stroke-color: white;
  --vis-xy-label-stroke-width: 1px;
}

.stackBarLabels g[alert][cluster] {
  --vis-xy-label-stroke-width: 2px;
}

[reb-dev]

@rokotyan Should we also include CSS variables for dark theme?

[rokotyan]

@reb-dev Good point, I was also thinking about it. But I'm planning some additional work for this component soon, so we can cover it then.

[reb-dev]

👍