Add support for transition in CSS api.

[Ayfri]

This PR adds a builder to create transitions with the CSS API.

[eymar]

Hi @Ayfri, Thank you for your contribution!

Could you please add unit tests for all new APIs in .../web/core/src/jsTest? Look at TransformTests as an example. We simply verify that the built css string is correct.

Also, since this a new API, we usually mark it with @ExperimentalComposeWebStyleApi annotation. It needs to be added to all new classes and extension functions.

P.S. I apologise for a long time without any answer

[Ayfri]

I'm creating the unit tests and facing a problem where we have to decide how the API behave :

transitions {
	duration(1.s)
	delay(.5.s)
	properties("height", "width")
	
	"width" {
		duration(2.s)
	}
}

Should it returns

1. height 1s 0.5s, width 2s by replacing the width property.
2. height 1s 0.5s, width 2s 0.5s by modifying the width property.
3. height 1s 0.5s, width 1s 0.5s, width 2s by just adding the property, behaves same as 1 but has a repetition.

Currently, it has the third behavior.

[eymar]

@Ayfri

3. height 1s 0.5s, width 1s 0.5s, width 2s by just adding the property, behaves same as 1 but has a repetition.

I think 3rd variant is okay, since it's possible to write such a css manually as well.