Async load json

[scambier]

• Added .loadJSONAsync() and .loadJSAsync() that are counterparts to their non-async version
• Added a simple test to make sure that .loadJSONAsync() creates a MiniSearch instance that is identical to .loadJSON()
• Replaced a few import with import type because TypeScript wasn't happy when I imported the project as a local dependency

I tried to dedupe the code as much as possible. However, the several await calls in inner loops prevent to do this cleanly without reducing code readability.

[lucaong]

It looks great! I will merge this tomorrow, as now it's late here and I want to give it one last look with fresh eyes.