Reaction to fs.promises API

[segayuu]

Node.js made fs.promises API stable when version12 became LTS. Since it is backported to v10 at the same time, fs.promises can be used stably in v10, v12, and v13.
The bluebird library authors recommend using bluebird only around the year "when you need more performance than collection methods and asynchronous features" around 2017.
Most hexo-fs features are now available with the fs.promises API.
I think we are forced to make a choice regarding the policy of this package.

• Maintaining compatibility
• Reimplement based on fs.promises API
• Encourage package users to move to fs.promises

Or there may be other options, and there may be a compromise.
However, there are some implementations such as fs.Direct that run faster than the implementation of this library.