Harmonise hover interaction across chart types (where sensible)

[sophiamersmann]

Closely related to #3918, #3381 and #4018

• On hover, some chart types increase the opacity of the hovered element, while others brighten the element's colour (Marimekko) or de-emphasize non-hovered elements in different ways (either by reducing the opacity or by using grey)
• Thinking about selection states or thinking about the opacity of chart elements also forces us to think about hover interactions. These issues should probably be solved in tandem

Designs: https://www.figma.com/design/2tH8dIkXurkktvumTneZZq/Grapher-UI-1.1?node-id=529-1204&t=FDZiC27W8K1h4kvT-1

Chart types:

• LineChart
• StackedBar
• ✨ (stacked area) improve hover states / TAS-687 #4129
• SlopeChart
• DiscreteBar
• StackedDiscreteBar
• ScatterPlot
• Marimekko
• MapChart

[marcelgerber]

This could be a nice stretch goal for #3381.

There are most likely some good reasons for some of these decisions that have been made in the past, but it is good to have another critical look at them and see if we want to reconsider them now.

[sophiamersmann]

Marwa's designs and bullet point lists of low-hanging fruit: https://www.figma.com/design/2tH8dIkXurkktvumTneZZq/Grapher-UI-1.1?node-id=529-1204&t=FDZiC27W8K1h4kvT-1