break: add "exports" map for native ESM support (#6)

* Support importing totalist/sync from es modules

Currently when I'm trying to import `totalist/sync` from
es module in node I get this error

```
Error [ERR_UNSUPPORTED_DIR_IMPORT]: Directory import '.../node_modules/totalist/sync' is not supported resolving ES modules imported from my-script.js
Did you mean to import totalist/sync/index.js?
```

Exports field will help to fix this.

* Update package.json

Co-authored-by: Luke Edwards <[email protected]>

package.json

@@ -6,6 +6,16 @@
   "module": "dist/index.mjs",
   "main": "dist/index.js",
   "types": "index.d.ts",
+  "exports": {
+    ".": {
+      "import": "./dist/index.mjs",
+      "require": "./dist/index.js"
+    },
+    "./sync": {
+      "import": "./sync/index.mjs",
+      "require": "./sync/index.js"
+    }
+  },
   "license": "MIT",
   "files": [
     "index.d.ts",