refactor: add tsc watch script

This commit adds the `tsc:watch` script that can be used to compile the
typescript code in watch mode.

package.json

@@ -6,6 +6,7 @@
   "main": "lib/main.js",
   "scripts": {
     "build": "tsc",
+    "build:watch": "tsc --watch",
     "format": "prettier --write '**/*.ts'",
     "format-check": "prettier --check '**/*.ts'",
     "lint": "eslint src/**/*.ts",