[css-text] Segment Break Transformation Rules at the beginning/end

[kojiishi]

Segment Break Transformation Rules defines rules based on the character before and after, but does not define when a segment break is at the beginning/end.

I assume it should be removed too, or Phase I: Collapsing and Transformation should remove leading/trailing space/tab/segment breaks?

/cc @fantasai @upsuper

[upsuper]

I don't think it really matters, because they should be trimmed in Phase II anyway.

[kojiishi]

Thanks, I remember we had that somewhere but could not find it, thank you for pointing this out.