Use plugins_url() to allow Workbox script URLs to be filtered

[westonruter]

Fixes #543.

Let's say you have all of the PWA plugin's files located at a CDN with the URL https://pwa-wp-cdn.example.com/. You could achieve that with the following plugin code:

if ( defined( 'PWA_PLUGIN_FILE' ) ) {
	add_filter(
		'plugins_url',
		static function ( $url, $path, $plugin ) {
			if ( PWA_PLUGIN_FILE === $plugin ) {
				$url = "https://pwa-wp-cdn.example.com/{$path}";
			}
			return $url;
		},
		10,
		3
	);
}

[codecov-commenter]

Codecov Report

Merging #600 (df1598b) into update/skip-waiting-message (49603fe) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@                        Coverage Diff                        @@
##             update/skip-waiting-message     #600      +/-   ##
=================================================================
- Coverage                          16.78%   16.74%   -0.05%     
  Complexity                           305      305              
=================================================================
  Files                                 55       55              
  Lines                               2008     2013       +5     
=================================================================
  Hits                                 337      337              
- Misses                              1671     1676       +5     

Flag	Coverage Δ
php	16.74% <0.00%> (-0.05%)	⬇️
unit	16.74% <0.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
...lass-wp-service-worker-configuration-component.php	0.00% <0.00%> (ø)
wp-includes/service-workers.php	18.96% <0.00%> (-0.68%)	⬇️

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