[css-easing-1] Clarify computed values and serialization

The CSS Easing spec has a section on serialization, however this section doesn't distinguish between specified and computed values (and seems to be defining the computed value serialization). Should there be any differences between these two? E.g.

• Should specified values of step-end and step-start be serialized as themselves rather than as a steps() function?
• Should the jump-end and end keywords be kept as distinct specified values?

Relatedly, the spec seems a bit confused about the computed/serialized values for start/end/jump-start/jump-end: the serializations of the “end” values are collapsed to a single serialization, but not the “start” values. I think

• jump-start/jump-end should compute to start/end or vice versa (unsure which we prefer).
• the computed value serialization should fall out from that

Start with the CSS Easing Level 1 serialization section linked in the issue. Compare its treatment of specified values, computed values, and serialization for step-start, step-end, start, end, jump-start, and jump-end. Done means the specification clearly distinguishes these stages and resolves the keyword relationships and serialization behavior.