Add support for Void? flags

Sources/ArgumentParser/Parsable Properties/Flag.swift

@@ -144,6 +144,17 @@ extension Flag where Value == Optional<Bool> {
   }
 }
 
+extension Flag where Value == Optional<Void> {
+  public init(
+    name: NameSpecification = .long,
+    help: ArgumentHelp? = nil
+  ) {
+     self.init(_parsedValue: .init { key in
+      .flag(key: key, name: name, help: help, value: ())
+    })
+  }
+}
+
 extension Flag where Value == Bool {
   /// Creates a Boolean property that reads its value from the presence of a
   /// flag.

Sources/ArgumentParser/Parsing/ArgumentSet.swift

@@ -104,6 +104,14 @@ extension ArgumentSet {
     return ArgumentSet(arg)
   }
 
+  static func flag(key: InputKey, name: NameSpecification, help: ArgumentHelp?, value: Void) -> ArgumentSet {
+    let help = ArgumentDefinition.Help(options: .isOptional, help: help, key: key)
+    let arg = ArgumentDefinition(kind: .name(key: key, specification: name), help: help, update: .nullary({ (origin, name, values) in
+      values.set(value, forKey: key, inputOrigin: origin)
+    }))
+    return ArgumentSet(arg)
+  }
+
   static func updateFlag<Value: Equatable>(key: InputKey, value: Value, origin: InputOrigin, values: inout ParsedValues, hasUpdated: Bool, exclusivity: FlagExclusivity) throws -> Bool {
     switch (hasUpdated, exclusivity) {
     case (true, .exclusive):

Tests/ArgumentParserEndToEndTests/FlagsEndToEndTests.swift

@@ -330,3 +330,23 @@ extension FlagsEndToEndTests {
     }
   }
 }
+
+
+fileprivate struct VoidFlag: ParsableArguments {
+  @Flag()
+  var test: Void?
+}
+
+extension FlagsEndToEndTests {
+  func testParsingVoid_DefaultNil() throws {
+    AssertParse(VoidFlag.self, []) { options in
+      XCTAssert(options.test == nil)
+    }
+  }
+
+  func testParsingVoid_OverrideDefault() throws {
+    AssertParse(VoidFlag.self, ["--test"]) { options in
+      XCTAssert(options.test != nil)
+    }
+  }
+}