feat: add json2md.async method for async converters

[adoyle-h]

• json2md.async will return a Promise for asynchronous handle

[IonicaBizau]

Sorry if I'm missing anything, but isn't this still sync?

[adoyle-h]

The func can be either asynchronous or synchronous function.

For examples:

json2md.converters.md = async (filepath) => {
    return await Promise.fromCallback((cb) => FS.readFile(filepath, {encoding: 'utf8'}, cb));
};

json2md.converters.asyncResult = () => Promise.resolve('async');

[IonicaBizau]

I love the idea. Thanks for this!

However, I'm not sure what async would mean in this case. Just because sync functions are wrapped around in promises, doesn't make them really async.

[adoyle-h]

Yes, sync functions could be wrapped around in promises. json2md.async return a Promise, so it is different from json2md function signature. The new entrypoint just for package version compatibility.

[adoyle-h]

Any problems? @IonicaBizau

[IonicaBizau]

Um, I see your point, but in what context would any of the converter functions be async? They are supposed to be small, sync functions.

[adoyle-h]

I want to read content from file, database, CDN or other services, not only transform content in converter. A part of content may be fetched from the outside of process rather than static variables in codes. For this scenario, the data passed to json2md is not pure static data, but a condition for fetching and transforming data. json2md.async([{type: data}, {type: condition}])
Although NodeJS provides some sync functions such as fs.readdirSync and fs.readFileSync, I think any interactions with the disk and network should be async.
It will be more flexible for user if converter could support both async and sync functions.

[IonicaBizau]

Ah, I understand now. 🚀

Can you add some tests before merging?

[adoyle-h]

OK. It may take a little time.

[adoyle-h]

@IonicaBizau done.