Set showHelp return type to never

[ryanrhee]

Since showHelp() calls into process.exit(), they should use the same return type.

The return type of process.exit() is never, not void. This tells the typescript compiler that code branches that have showHelp() terminate, and that any code written as executing after showHelp() should be considered dead code (similar to code after a return statement).

[ryanrhee]

As a workaround:

  const foo: string | undefined = "foo"

  function showHelp(): never {
    cli.showHelp()
    throw new Error("This is unreachable code")
  }
  
  if (foo === undefined) {
    showHelp()
  }
  
  // type of foo is now `string`