WordPress: Add theme template from plugin if it doesn't exist in the theme folder.

template_include.php

<?php
add_filter( 'template_include', 'my_prefix_template' );
function my_prefix_template( $template ) {

  $template_filename = "foobar.php";

  if ( !file_exists( get_stylesheet_directory() . "/" . $template_filename ) ) {
    $template = plugin_dir_path( __FILE__ ) . '/' . $template_filename;
  }
  
  return $template;
}