validate can get a Promise

[rluvaton]

Is your feature request related to a problem?

I can't use validate that returns a Promise.

Use cases which this would be helpful:

• Check if directory/file exists
• Read from DB
• Do some async request

Describe the solution you'd like

I would like to be able to pass a function that returns Promise and when the promise fails or returns a string it will indicate an invalid value.

Describe alternatives you've considered

deasync - Anti pattern

Additional context

Implementation suggestions:

1. Check if the return value is Promise by checking if .then exist
2. Transform any value to Promise and handle it everywhere like Promise:

// Option 1.1:
function option1_1() {
   return Promise.resolve(validator(args)) 
}

// Option 1.2:
function option1_2() {
   const validatorRes = validator(args);

   if(validatorRes.then) {
      return validator;
   }

   if(validatorRes === true) {
      return Promise.resolve(validatorRes);
   }

   return Promise.reject(validatorRes);
}


// Option 2.1:

function option2_1() {
  return new Promise(resolve => {
      // If the validator function throws it will reject the promise
      resolve(validator(args));
   });
}

// Option 2.2:
function option2_2() {
   return new Promise(resolve => {
      // If the validator function throws it will reject the promise
      resolve(validator(args));
   }).then(res => {
      if(res === true) {
         return true;
      }
      throw new Error(res);
   });
}

[rluvaton]

It looks like it can get a promise, just need to update the docs to say that.

[DenyVeyten]

It has an issue for Windows described in #402

Also, the issue might be resolved by this PR #313, but it looks like this repo has not been maintained for 2 years.

But here is another issue for async validation:

prompts/lib/index.js

Line 22 in 7c16709

if (!skipValidation && question.validate && question.validate(answer) !== true) {

There should be await question.validate(answer)