Add async/defer tags for enqueueing scripts

[colorful-tones]

Fixes #51

I'm not sure how we want to handle documentation/recommendations for usage?

Example, in order to use you would put something like this in current Theme Scaffold's includes/core.php:

Currently:

function scripts() {

	wp_enqueue_script(
		'frontend',
		TENUP_SCAFFOLD_TEMPLATE_URL . '/dist/js/frontend.min.js',
		[],
		TENUP_SCAFFOLD_VERSION,
		true
	);

}

Recommended Usage:

function scripts() {

	wp_enqueue_script(
		'frontend',
		TENUP_SCAFFOLD_TEMPLATE_URL . '/dist/js/frontend.min.js',
		[],
		TENUP_SCAFFOLD_VERSION,
		true
	);

	// Should you prefer to load frontend.min.js async
	wp_script_add_data(
		'frontend', 
		'script_execution', 
		'async' 
	);

}

[timwright12]

I'm going to kick this one over to @daveross as the backender. This is the article from the issue: https://matthewhorne.me/defer-async-wordpress-scripts/

[timwright12]

Let's also make sure this gets into the plugin repo once this is all worked out. I'll create an issue for it.

[colorful-tones]

Pull Request was submitted on Theme Scaffold to mirror this one.

[dana-ross]

👍 This works nicely. Thanks for taking the time to put this together and extra thanks for using a mechanism WordPress already provides (wp_script_add_data).