Make Node.js treat files with the extension .js as ECMAScript modules

Although most of the code will be TypeScript and the transpiler could
emit CommonJS JavaScript, there will be some JavaScript: the new flat
configuration file for ESLint.

must be done now, before setting up Yarn, for Yarn to detect that it
should enable ESM support (¿only in the Plug'n'Play loader or elsewhere
too?)

package.json

@@ -2,5 +2,6 @@
 	"name": "@djsp/aws-organization-bootstrap",
 	"version": "0.0.0",
 	"files": [
-	]
+	],
+	"type": "module"
 }