Updates swift_argument_parser and marks it as a dev devependency

[csmulhern]

This cleans up some warnings from the earlier version of the repo, though unfortunately it adds a couple of others.

It also marks swift_argument_parser as a dev dependency, as it's only used in tests.

Before:

[#DeprecatedDeclaration]: <https://docs.swift.org/compiler/documentation/diagnostics/deprecated-declaration>
INFO: From Compiling Swift module @@swift_argument_parser+//:ArgumentParser [for tool]:
external/swift_argument_parser+/Sources/ArgumentParser/Parsable Types/CommandConfiguration.swift:50:14: warning: stored property 'subcommands' of 'Sendable'-conforming struct 'CommandConfiguration' contains non-Sendable type 'any ParsableCommand.Type'
 48 |
 49 |   /// An array of the types that define subcommands for this command.
 50 |   public var subcommands: [ParsableCommand.Type]
    |              `- warning: stored property 'subcommands' of 'Sendable'-conforming struct 'CommandConfiguration' contains non-Sendable type 'any ParsableCommand.Type'
 51 |
 52 |   /// The default command type to run if no subcommand is given.

external/swift_argument_parser+/Sources/ArgumentParser/Parsable Types/CommandConfiguration.swift:53:14: warning: stored property 'defaultSubcommand' of 'Sendable'-conforming struct 'CommandConfiguration' contains non-Sendable type 'any ParsableCommand.Type'
 51 |
 52 |   /// The default command type to run if no subcommand is given.
 53 |   public var defaultSubcommand: ParsableCommand.Type?
    |              `- warning: stored property 'defaultSubcommand' of 'Sendable'-conforming struct 'CommandConfiguration' contains non-Sendable type 'any ParsableCommand.Type'
 54 |
 55 |   /// Flag names to be used for help.

external/swift_argument_parser+/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift:26:7: warning: stored property 'parsableArgumentsType' of 'Sendable'-conforming struct 'ParsableArgumentsValidationError' has non-Sendable type 'any ParsableArguments.Type'
 24 |
 25 | struct ParsableArgumentsValidationError: Error, CustomStringConvertible {
 26 |   let parsableArgumentsType: ParsableArguments.Type
    |       `- warning: stored property 'parsableArgumentsType' of 'Sendable'-conforming struct 'ParsableArgumentsValidationError' has non-Sendable type 'any ParsableArguments.Type'
 27 |   let underlayingErrors: [Error]
 28 |   var description: String {

external/swift_argument_parser+/Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift:177:9: warning: stored property 'type' of 'Sendable'-conforming struct 'InvalidDecoderError' has non-Sendable type 'any ParsableArguments.Type'
175 |
176 |   struct InvalidDecoderError: ParsableArgumentsValidatorError, CustomStringConvertible {
177 |     let type: ParsableArguments.Type
    |         `- warning: stored property 'type' of 'Sendable'-conforming struct 'InvalidDecoderError' has non-Sendable type 'any ParsableArguments.Type'
178 |
179 |     var description: String {

external/swift_argument_parser+/Sources/ArgumentParser/Parsing/CommandParser.swift:13:7: warning: stored property 'commandStack' of 'Sendable'-conforming struct 'CommandError' contains non-Sendable type 'any ParsableCommand.Type'
 11 |
 12 | struct CommandError: Error {
 13 |   var commandStack: [ParsableCommand.Type]
    |       `- warning: stored property 'commandStack' of 'Sendable'-conforming struct 'CommandError' contains non-Sendable type 'any ParsableCommand.Type'
 14 |   var parserError: ParserError
 15 | }

external/swift_argument_parser+/Sources/ArgumentParser/Utilities/Tree.swift:102:10: warning: associated value 'recursiveSubcommand' of 'Sendable'-conforming enum 'InitializationError' has non-Sendable type 'any ParsableCommand.Type'
100 |
101 |   enum InitializationError: Error {
102 |     case recursiveSubcommand(ParsableCommand.Type)
    |          `- warning: associated value 'recursiveSubcommand' of 'Sendable'-conforming enum 'InitializationError' has non-Sendable type 'any ParsableCommand.Type'
103 |   }
104 | }

After:

external/swift_argument_parser+/Sources/ArgumentParser/Parsable Types/ParsableArguments.swift:76:8: warning: '_errorLabel' is deprecated: Use _errorPrefix instead. [#DeprecatedDeclaration]
 74 |   /// Defaults to `"\(_errorLabel): "`.
 75 |   public static var _errorPrefix: String {
 76 |     "\(_errorLabel): "
    |        `- warning: '_errorLabel' is deprecated: Use _errorPrefix instead. [#DeprecatedDeclaration]
 77 |   }
 78 | }

external/swift_argument_parser+/Sources/ArgumentParser/Usage/DumpHelpGenerator.swift:209:15: warning: 'customDeprecated' is deprecated: Use custom instead. [#DeprecatedDeclaration]
207 |       self = .customAsync
208 |     case .customDeprecated(_):
209 |       self = .customDeprecated
    |               `- warning: 'customDeprecated' is deprecated: Use custom instead. [#DeprecatedDeclaration]
210 |     }
211 |   }

[aaronsky]

afaik test_discoverer isn’t a dev dependency tool