Fix async middleware

[jly36963]

Addresses: #2124

Description

The result of applyMiddleware is Arguments | Promise<Arguments>, but the code treats it synchronously. Whenever it returns a promise, that promise isn't handled as such.

This problem is easier to spot when strict is used

Reproduction

const input = 'cmd1 -f Hello -b world'

yargs(input)
  .command(
    'cmd1',
    'cmd1 desc',
    yargs => yargs
      .option('foo', { type: 'string', alias: 'f', required: true })
      .option('bar', { type: 'string', alias: 'b', required: true })
      .middleware(async argv => {
        console.log('sleeping')
        await new Promise(r => setTimeout(r, 2000))
        console.log('waking up')
        return argv
      }, true),
    argv => {
      console.log(argv)
    })
  .strict()
  .parse()

I put a console log after applyMiddleware is called, and I could see that 'waking up' showed up after applyMiddleware finished.

[bcoe]

I would be tempted to write a more specific test to #2124

"Using the strict option in combination with an async middleware, which is applied before the validation, does not result in unknown command"

[jly36963]

I simplified the test. I kept the done logic, because Mocha will pass this test before the unhandled promise would cause it to fail. If there's a regression, this test will fail because done will get called twice, and one of the executions will have an error passed as an argument

[bcoe]

With a couple nits.

[bcoe]

I had to read this a few times, I'm wondering if it would be worth simplifying to exactly the minimal case outlined in the regression:

Yargs
  .strict(true)
  .middleware(async () => {}, true)
  .command("test", false, () => {}, () => {})
  .parseAsync(['test']);

I don't have a strong opinion though, if your goal was to exercise parts of the codebase that weren't covered in the above example.