Add explicit .js extension for Node16 module resolution

Author: tspng

Gruntfile.cjs

@@ -114,7 +114,7 @@ module.exports = function(grunt) {
 	// build tom-select.custom.js
 	var plugin_arg			= grunt.option('plugins');
 	var custom_file			= path.resolve( process.cwd(),'./src/tom-select.custom.ts');
-	var custom_content		= ['/* this file is generated by grunt when calling `npm run build -- --plugins=<plugins> */','import TomSelect from "./tom-select";'];
+	var custom_content		= ['/* this file is generated by grunt when calling `npm run build -- --plugins=<plugins> */','import TomSelect from "./tom-select.js";'];
 
 	if( fs.existsSync(custom_file) ){
 		fs.unlink(custom_file,err => {