Add ways to configure EasingFunction::Steps via new StepConfig

[RobWalt]

Objective

• In Fix rounding in steps easing function #17743, attention was raised to the fact that we supported an unusual kind of step easing function. The author of the fix kindly provided some links to standards used in CSS. It would be desirable to support generally agreed upon standards so this PR here tries to implement an extra configuration option of the step easing function
• Resolve Support more "step" easing modes #17744

Solution

• Introduce StepConfig
• StepConfig can configure both the number of steps and the jumping behavior of the function
• StepConfig replaces the raw usize parameter of the EasingFunction::Steps(usize) construct.
• StepConfigs default jumping behavior is end, so in that way it follows Fix rounding in steps easing function #17743

Testing

• I added a new test per JumpAt jumping behavior. These tests replicate the visuals that can be found at https://developer.mozilla.org/en-US/docs/Web/CSS/easing-function/steps#description

Migration Guide

• EasingFunction::Steps now uses a StepConfig instead of a raw usize. You can replicate the previous behavior by replaceing EasingFunction::Steps(10) with EasingFunction::Steps(StepConfig::new(10)).