Add Asset Manager

[QWp6t]

I might rework the config a bit. Right now it's set up in such a way as to make it easy to add multiple manifests via config, but I'm not sure how frequently that would actually get utilized.

I borrowed the term "strategy" from Symfony. I like it. ¯\_(ツ)_/¯

The @asset() directive and asset() helper function both accept a 2nd parameter now that specifies the name of a manifest. So if a plugin registers a manifest, for example, that plugin can now use those helpers by specifying the name used in registering.

// register the 'my-plugin' manifest
app('assets')->register('my-plugin', $manifest);

{{-- access 'my-plugin' via @asset() directive --}}
@asset('images/logo.png', 'my-plugin')

// access 'my-plugin' manifest via asset() helper function
wp_enqueue_script('my-plugin', asset('scripts/bundle.js', 'my-plugin')->uri(), null, null, true);