Introduce an add_editor_style function

[kadamwhite]

add_editor_style takes a theme-relative path to a stylesheet, and enqueues that stylesheet as an editor asset. Given that hashed filenames can make it challenging to use editor stylesheets, we should consider introducing a helper method which can take the name of a bundle in the manifest and ensure it gets ingested as an editor stylesheet.

Pseudo-code from #35 which we'd want to wrap in an Asset_Loader\add_editor_style() top-level function:

	$editor_stylesheet = Asset_Loader\Manifest\get_manifest_resource(
		get_theme_file_path( 'dist/production-asset-manifest.json' ),
		'tetramino.css'
	);

	add_editor_style( "dist/$editor_stylesheet" );

[kadamwhite]

@Sephsekla Thoughts on whether this would be useful in a 1.0? Re: #40

[goldenapples]

I think this would be more useful if we could get something like humanmade/webpack-helpers#7 finalized, and the time might be right to do that now?

[kadamwhite]

Noting a rough implementation of how the manifest is manually read and the specified asset enqueued on another project

	$json = file_get_contents( $manifest );
	$json_data = json_decode( $json, true );

	if ( isset( $json_data['editor.css'] ) ) {
		add_editor_style( __DIR__ . "/../../build/{$json_data['editor.css']}" );
	}