ESM support for Node

[stalniy]

Node 14 supports ESM out of the box and dual package loading, so what I need to do is to add exports field to every package. This is a new way to point to package entry points. While the main file of the module uses CommonJS, I need to use mjs extension for all ESM files. This is important, otherwise Node thinks that the file uses commonjs even in mjs file.

Example for @casl/ability

{
  "exports": {
    ".": {
      "import": "./dist/es6/index.mjs",
      "require": "./dist/umd/index.js"
    },
    "./extra": {
      "import": "./dist/es6/extra.mjs",
      "require": "./dist/umd/extra.js"
    }
  }
}

The same needs to be done for sift.js, otherwise the package cannot be loaded