chore: add export maps to support native ESM (#69)

Fixes #68

Author: layershifter

package.json

@@ -6,6 +6,30 @@
   "jsnext:main": "dist/esm/index.js",
   "module": "dist/esm/index.js",
   "types": "types.d.ts",
+  "exports": {
+    ".": {
+      "types": "./types.d.ts",
+      "node": "./dist/cjs/index.js",
+      "import": "./dist/esm/index.js",
+      "default": "./dist/cjs/index.js"
+    },
+    "./core": {
+      "types": "./core.d.ts",
+      "node": "./dist/cjs/core.js",
+      "import": "./dist/esm/core.js",
+      "default": "./dist/cjs/core.js"
+    },
+    "./core.esm": {
+      "default": "./dist/esm/core.js"
+    },
+    "./core.esm.js": {
+      "default": "./dist/esm/core.js"
+    },
+    "./core.js": {
+      "default": "./dist/cjs/core.js"
+    },
+    "./package.json": "./package.json"
+  },
   "scripts": {
     "build": "rimraf dist && npm-run-all build:**",
     "build:bundlers": "kcd-scripts build --bundle cjs,esm --environment BUILD_INPUT:src/*.js --no-clean",