Mark CommandConfiguration as Sendable (#615)

Lack of this marker protocol conformance causes warnings with strict concurrency checks, such as `Static property 'configuration' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6`

Author: MaxDesiatov

Sources/ArgumentParser/Parsable Types/CommandConfiguration.swift

@@ -10,7 +10,7 @@
 //===----------------------------------------------------------------------===//
 
 /// The configuration for a command.
-public struct CommandConfiguration {
+public struct CommandConfiguration: Sendable {
   /// The name of the command to use on the command line.
   ///
   /// If `nil`, the command name is derived by converting the name of