build: add fixup script for dual modules

See:
- microsoft/TypeScript#18442 (comment)
- https://www.sensedeep.com/blog/posts/2021/how-to-create-single-source-npm-module.html

Author: Zolboobayar Gantumur

fixup.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+cat >lib/cjs/package.json <<!EOF
+{
+    "type": "commonjs"
+}
+!EOF
+
+cat >lib/esm/package.json <<!EOF
+{
+    "type": "module"
+}
+!EOF

package.json

@@ -24,7 +24,7 @@
   "scripts": {
     "build:esm": "tsc -p tsconfig.build.json",
     "build:cjs": "tsc -p tsconfig.cjs.json",
-    "build": "npm run build:cjs && npm run build:esm",
+    "build": "npm run build:cjs && npm run build:esm && ./fixup.sh",
     "test": "jest",
     "test-e2e": "jest --config ./e2e/jest.config.js",
     "lint": "eslint src/**/*.ts"