Add native ESM entry point

[marvinhagemeister]

This PR adds an ESM entry point so that this library can be used with ES modules natively in node.

Explanation:

"exports": {
  // Main/default entry point
  ".": {
    "browser": "./dist/sourcemap-codec.umd.js",
    // Must be `.mjs` because our `package.json` doesn't include `"type": "module"`
    "import": "./dist/sourcemap-codec.mjs",
    // CommonJS entry point
    "require": "./dist/sourcemap-codec.umd.js"
  },
  // If any module wants to import `sourcemap-codec/package.json` for some reason
  // (grabbing package.version or something)
  "./package.json": "./package.json",
  // Old-style deep imports, like `require("sourcemap-codec/dist/sourcemap-codec.umd.js")`
  "./": "./"
},

Verified both CommonJS and ESM entries in node 12.18.0 locally.

Fixes #81 .

[wangjia184]

when can this PR be released in npm?