add: unit and integration testing

app/node_modules/import-local/index.d.ts

@@ -0,0 +1,17 @@
+/**
+Let a globally installed package use a locally installed version of itself if available.
+
+@param filePath - The absolute file path to the main file of the package.
+
+@example
+```
+import importLocal from 'import-local';
+
+if (importLocal(import.meta.url)) {
+	console.log('Using local version of this package');
+} else {
+	// Code for both global and local version here…
+}
+```
+*/
+export default function importLocal(filePath: string): boolean | undefined | unknown;