feat: support `Duration` for CSS properties instead of `double`

[ShubhamVG]

Description

It is confusing whether the duration in things like Transition is in seconds or milliseconds, and dart has Duration so it makes sense to use it.

I propose two things:

• Duration to be used Transition.
• DurationExt (just like numstr) to convert that Duration's value into milliseconds.

Additional Context

BTW, I can work on this.

[schultek]

Great idea! Feel free to create a PR.