Support async custom completion functions for AsyncParsableCommand via async/await

[rgoldberg]

• Support async custom completion functions for AsyncParsableCommand via async/await.
  • Resolve Hangs for async custom completions #852.
  • No longer use DispatchSemaphore, so more modern standards compliant & will work for platforms that do not have DispatchSemaphore.
  • The existing DispatchSemaphore implementation hangs when any async custom completions run if the root command is a sync ParsableCommand; async custom completions do not hang because of DispatchSemaphore if the root command is an AsyncParsableCommand, regardless if the command containing the property for the completing argument is a sync ParsableCommand or an AsyncParsableCommand.
  • Async custom completions are thus not supported for root ParsableCommand.
• Make some related code more concise & consistent.
• Improve ParsableArguments#_errorLabel DocC (unrelated, but just noticed it should be improved).

A few potential issues with / questions about what you want from this PR:

• Async custom completions are supported only for AsyncParsableCommand root commands.
  • That seems OK to me because:
    • The existing implementation doesn't support root sync ParsableCommand.
    • I doubt there's any penalty to switch a root command from ParsableCommand to AsyncParsableCommand, since Swift Concurrency must be available, since the code is trying to use an async function for completion.
• If the root command is a sync ParsableCommand, a ParserError.invalidState is thrown during runtime when completion candidates are requested from an async custom completion function.
  • Do you want this detected at build time or completion script generation time?
    • If so, are there any utilities to facilitate detection at build time or completion script generation time?
  • Do you want a different error?
• Is the minor cleanup for tangentially related code acceptable here?
• Is the unrelated DocC update acceptable here?

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary