[PROPOSED] Unify CustomParser and DefaultValueFactory

[jonsequitur]

Prior to the release of System.CommandLine 2.0.0, we found a number of things about the Argument/Option.CustomParser and Argument/Option.DefaultValueFactory API design to be awkward.

They have nearly identical signatures but one (DefaultValueFactory) is called only when no tokens are matched for the symbol, while the other (CustomParser) is called only when tokens are matched for the symbol. This was a regression from the original design where custom parsers were able to fulfill both of these roles at once. With the current API, if you want to use the same logic for both of these cases, you need to assign the same delegate to both properties.

The goal of this proposed API is to unify this functionality and, hopefully, to make the usage clearer.

Please let me know what you think. Alternative proposals are welcome. Consider this PR an example to start conversation over how to improve this area of the System.CommandLine API.

[jonsequitur]

This clearly needs a better name. It indicates when the value factory should be called, based on whether or not a symbol has matching tokens.

[KalleOlaviNiemitalo]

I find it handy that I can have a custom parser dedicated to a type but agnostic of default values; that also lets me use the same parser for multiple options with different default values. If the properties were merged again, then I'd instead have to make the parser method take the default value as a parameter, so its signature would no longer match that of the delegate.