Fix breaking changes from the color function API

[rokotyan]

f5#856

This PR addresses two breaking changes introduced by the recent custom color function work, before they ship in a release.

Rename color to colorFunction

The new custom color function config property on containers and BulletLegend was named color, which is both ambiguous (it holds a function, not a color value) and collides with the widely used color accessor naming on components. It is now colorFunction:

// Before
<VisXYContainer color={myColorScale}>
// After
<VisXYContainer colorFunction={myColorScale}>

Applied to ContainerConfigInterface (so both SingleContainer and XYContainer), BulletLegendConfigInterface, and the Angular wrappers — which also now actually expose the input on VisSingleContainerComponent and VisXYContainerComponent. Dev examples and website docs (Single_Container, XY_Container, theming guide) are updated to match.

Backward compatibility for getColor

The getColor utility's fourth positional argument used to be dontFallbackToCssVar: boolean and was repurposed as key: string, silently breaking existing callers. getColor now accepts string | boolean there: a string is treated as a color key, while true is interpreted as the legacy dontFallbackToCssVar flag (unless explicitly set via options). Marked with TODOs to drop the boolean form in Unovis 2.0.