Force script to exit upon successful completion (#26)

For as-yet-unknown reasons, we've experienced a few runs that "hang" after the `Process complete.` line is output.

Author: ethitter

classes/class-init.php

@@ -94,6 +94,7 @@ public function start( array $args, array $assoc_args ): void {
 			}
 
 			WP_CLI::line( 'Process complete.' );
+			exit( 0 );
 		} catch ( Throwable $throwable ) {
 			$this->_handle_error( $throwable );
 		}