[Feature] timelineIndex in options.series

### What problem does this feature solve?

Currently, options.series does not support a `timelineIndex` property, making it impossible to configure multiple timelines independently. As a result, all timelines share the same series configurations, which limits flexibility when working with multiple timelines in the same chart.

Adding a `timelineIndex` property to options.series would allow each timeline to manage its own series separately, enabling more precise control over different dataset transitions. Right now, this is not possible with the existing API.

Would it be possible to support this feature?

### What does the proposed API look like?

I propose adding a `timelineIndex` property to options.series, allowing each series to be linked to a specific timeline. This would enable independent timeline configurations instead of forcing all timelines to share the same series settings.

const option = {
baseOption: {
timeline: [
{ axisType: "category", data: ["2020", "2021", "2022"] },
{ axisType: "value", data: [10, 20, 30] }
],
series: []
},
options: [
{
series: [
{
type: "line",
data: [120, 200, 150],
timelineIndex: 0 // Links this series to the first timeline
},
{
type: "bar",
data: [80, 180, 220],
timelineIndex: 1 // Links this series to the second timeline
}
]
},
{
series: [
{
type: "line",
data: [180, 250, 190],
timelineIndex: 0
},
{
type: "bar",
data: [50, 250, 110],
timelineIndex: 1
}
]
}
]
};