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)).

[alice-i-cecile]

I think the StepConfig is a bit convoluted; can you swap to EaseFunctionSteps(usize, JumpAt) to make this a bit nicer to use? I think that's a good compromise between the initial design and the one proposed in #17777.

[rparrett]

One other thing I appreciated about the other PR was the consistency with regard to the SVGs, generating them with the same tool that generated the others.

The SVGs in this PR are more informative, but they are also very large, and the style doesn't seem to fit quite as well.

Although in 17777, the svgs did not actually depict the separate step modes properly.

Expand StepConfig docs

Expand 17777 Step_ docs

[mockersf]

not sure those tests are really needed

[RobWalt]

Yeah it was more of an sanity check while developing the functions. They run in no time though (literally 0.0s on my machine xD) so it's not really a burden imo