[Feature] Tooltip shared option

### What problem does this feature solve?

When working with multiple series on spesifically line charts currently you can not disable shared tooltip. It always shows all series' tooltips. But sometimes you may want to use seperate tooltips. Making trigger option to 'item' does not work because it makes harder to hover a point. With this feature you should be able to use trigger:'axis' and show the closest value's tooltip on hover. Many chart libraries have this feature currently.

### What does the proposed API look like?

I should be able to hover somewhere in chart area (not just the points in datasets) it should find the closest value of the point i hover in dataset and show its tooltip. i should be able to control this with a parameter. if its true show both tooltips and its not the one i described above.

The issue does not name files, tests, or entry points. Start by locating the tooltip handling for line charts and the existing trigger option; done means a documented parameter can choose between shared tooltips and the closest hovered value while supporting hover anywhere in the chart area.

Written by the indexing model from the issue text.