Add both Module and CommonJS support

[yanickrochon]

I currently have an issue with this module in my project since we have upgraded Node.js to the latest stable version.

SyntaxError: The requested module 'lru_map' is expected to be of type CommonJS, which does not support named exports. CommonJS modules can be imported by importing the default export.
For example:
import pkg from 'lru_map';
const { LRUMap } = pkg;

The solution seems to be easy enough.

The CommonJS way

Rename the source to lru.mjs and fully support import/export, then transpile to CommonJS code to the dist folder. Then modify the package.json with the `exports" config :

   "main": "./dist/lru.js",
   "type": "commonjs",
   "exports": {
      ".": [
         {
            "import": "./lru.mjs",
            "require": "./dist/lru.js",
            "default": "./dist/lru.js"
         },
         "./dist/lru.js"
      ]
   }

The Module way

Do not rename the source file, but fully support import/export, then transpile to CommonJS code with the extension .cjs to the dist folder (i.e. ./dist/lru.cjs). The modify the package.json with the a similar exports config :

   "main": "./lru.js",
   "type": "module",
   "exports": {
      ".": [
         {
            "import": "./lru.js",
            "require": "./dist/lru.cjs",
            "default": "./lru.js"
         },
         "./lru.js"
      ]
   }

Tests

I tested this solution with both Node.js v12 and v14. It works with babel-node as well as without.