fix: isAsync is deprecated

[paullbn]

Hello,

This PR solves the following mongoose deprecation warning:

DeprecationWarning: Mongoose: the `isAsync` option for custom validators is deprecated.
Make your async validators return a promise instead: https://mongoosejs.com/docs/validation.html#async-custom-validators

Paul

[niftylettuce]

This actually doesn't need to be asynchronous at all nor return a Promise, I think we can just throw an error?

[codecov]

Codecov Report

Merging #7 into master will decrease coverage by 1.25%.
The diff coverage is 80%.

@@           Coverage Diff           @@
##           master    #7      +/-   ##
=======================================
- Coverage   81.25%   80%   -1.25%     
=======================================
  Files           1     1              
  Lines          48    50       +2     
=======================================
+ Hits           39    40       +1     
- Misses          9    10       +1

Impacted Files	Coverage Δ
index.js	80% <80%> (-1.25%)	⬇️

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[paullbn]

Ok, i thought there was a reason i didn't know that the validator was asynchronous with isAsync set to true, so i've just put it in a promise.

I'll update the PR as soon as i can

[luluhoc]

Will you update it?

[niftylettuce]

PR welcome!

[niftylettuce]

v2.0.2 has been released to npm with this fixed

npm install mongoose-slug-plugin@latest

or with yarn:

yarn add mongoose-slug-plugin@latest