upgrade to swift 6

[keyz]

$ make build
swift build -c release --disable-sandbox
Building for production...
/Users/keyz/dev/comicsans/Sources/CommandLineTool/cs.swift:60:30: warning: main actor-isolated instance method 'run()' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode
  5 |
  6 | @main
  7 | struct CommandLineTool: ParsableCommand {
    |                         `- note: add '@preconcurrency' to the 'ParsableCommand' conformance to defer isolation checking to run time
  8 |     static var configuration = CommandConfiguration(
  9 |         commandName: "cs",
    :
 58 |     }
 59 |
 60 |     @MainActor mutating func run() throws {
    |                              `- warning: main actor-isolated instance method 'run()' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode
 61 |         let text = try isExpectingPipe ? parsePipeInput() : text
 62 |

/Users/keyz/dev/comicsans/.build/checkouts/swift-argument-parser/Sources/ArgumentParser/Parsable Types/ParsableCommand.swift:32:17: note: mark the protocol requirement 'run()' 'async' to allow actor-isolated conformances
 30 |   /// This method has a default implementation that prints the help screen for
 31 |   /// this command.
 32 |   mutating func run() throws
    |                 `- note: mark the protocol requirement 'run()' 'async' to allow actor-isolated conformances
 33 | }
 34 |
[12/12] Linking cs
Build complete! (10.03s)