Migrate to TypeScript for better types

[Howard86]

Changes

• rewrite the library in ts
• add custom babel types and @mdx-js/mdx
• add custom next.config.js in __tests__ to fix fs module for testing, please refer to Module not found: Can't resolve 'fs' vercel/next.js#7755
• changed the import signature based on Rewrite in TypeScript hashicorp/next-mdx-remote#20 (comment) as the following

// currently
import hydrate from 'next-mdx-remote/hydrate'
import renderToString from 'next-mdx-remote/render-to-string'

// if we export all from `index.ts`
import {  hydrate, renderToString } from 'next-mdx-remote'

[06kellyjac]

Is this meant to be a PR into your fork's master or into hashicorp/next-mdx-remote's master?

[Howard86]

@06kellyjac
Oh, this is meant to be my own master. I am still testing the npm package myself and have found some weird bugs after my own rework. By the way, thank you for your comments about @babel/types! It does save me some time research what's behind the scene.

[Howard86]

next-mdx-remote has been greatly improved since, also shipped with types.
closed for now