[css-values-4] Serialization of Functional Notations

Tab and I noticed that we don't rigorously define the serialization of functional notations, so we're proposing to add a generic definition to CSS Values L4.

The non-controversial aspects are:

• Serialize the arguments per their individual grammars.
• Commas serialize with a following space.
• Space-separate adjacent tokens (except between a value and a following comma or closing parenthesis)

The one area where we have very little agreement is

• Whether to omit optional values in specified value serialization.

Generally-speaking, we omit optional components in computed values. This is an important principle of CSSOM serialization for computed values. But we are very inconsistent for specified values--this is extra confused by the fact that multiple "serialization" sections in our specs don't distinguish between specified and computed values and just assume there's only one serialization.

Start with the proposed generic definition for functional notations in CSS Values L4 and review the issue's discussion of specified versus computed value serialization. The non-controversial rules are listed, but completion requires resolving whether optional values should be omitted in specified value serialization and incorporating the agreed definition into the specification.