Add documentation for official workaround

[cseas]

This package isn't needed anymore with TypeScript now supporting .js file extensions in imports and a way to enforce them with a combination of the below configs.

microsoft/TypeScript#42813 (comment)

// package.json
{
  "type": "module"
}

// tsconfig.json
{
  "module": "NodeNext"
}

We do need to give up the ability to use extension-less imports but that's in line with how browsers and Node expect our code to be written anyway.

This package is still useful for someone who wants to avoid using file extensions or avoid dealing with all the problems that come with using type: module but as far as the ability to output valid ESM is concerned, the above configs do make it possible.