Feature request: easier access to function arguments

[nex3]

It's currently consistently annoying to access a list of arguments to a function call parsed by postcss-value-parser. It returns the children of the function as an undifferentiated list of nodes which may or may not have commas as part of the nodes, but in practice almost all CSS functions take comma-separated lists of arguments. It would be nice to have some kind of API to make it easier to access these, possibly as a list of ParsedValues.