Update System.CommandLine symbols API per API review notes

[jonsequitur]

The following API changes were agreed upon:

• CliSymbol.IsHidden => Hidden (drop the "Is")
• CliOption.AppliesToSelfAndChildren => Recursive
• CliOption.IsRequred = Required
• CliOption we added ValueType for symmetry with CliArgument
• DiagramDirective: errorExitCode was renamed to parseErrorReturnValue, and made a get/set property instead of a ctor parameter.
• We discussed the "Cli" prefix. We agree there does need to be some particle (because "Symbol", "Option", "Command", etc are too general and will probably have app collisions). No one thought it was great, but no one had a better name.

dotnet/runtime#68578 (comment)