WIP: add test coverage and fixes for #446

- Start of a fix for issue 446. After an initial read through the code
  it feels like there more @argument initializers than needed and this
  logic can probably be reudced and simplified.
- Fixes isOptional derivision for non-Optional @arguments with default
  values, however the simple solution doesn't work properly for Optional
  @arguments without a non-nil default value.

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -244,7 +244,11 @@ extension Argument {
     transform: @escaping (String) throws -> Value
   ) {
     self.init(_parsedValue: .init { key in
-      let help = ArgumentDefinition.Help(options: [], help: help, key: key)
+      let help = ArgumentDefinition.Help(
+        options: initial != nil ? [.isOptional] : [],
+        help: help,
+        defaultValue: initial.map { "\($0)" },
+        key: key)
       let arg = ArgumentDefinition(kind: .positional, help: help, completion: completion ?? .default, update: .unary({
         (origin, name, valueString, parsedValues) in
         do {

Tests/ArgumentParserUnitTests/HelpGenerationTests.swift

@@ -775,4 +775,28 @@ extension HelpGenerationTests {
         See 'custom-usage-hidden --help' for more information.
       """)
   }
+
+  struct ArgumentsWithTransform: ParsableCommand {
+    @Argument(help: "required", transform: { Int($0)! })
+    var arg0: Int
+    @Argument(help: "optional", transform: { Int($0)! })
+    var arg1: Int = 0
+    @Argument(help: "optional", transform: { Int($0)! })
+    var arg2: Int?
+  }
+
+  func testArgumentsWithTransform() {
+    AssertHelp(.default, for: ArgumentsWithTransform.self, equals: """
+        USAGE: arguments-with-transform <arg0> [<arg1>] [<arg2>]
+
+        ARGUMENTS:
+          <arg0>                  required
+          <arg1>                  optional (default: 0)
+          <arg2>                  optional
+
+        OPTIONS:
+          -h, --help              Show help information.
+
+        """)
+  }
 }