Support CSS-only entrypoints.

[kadamwhite]

Currently the way to support a CSS-only entrypoint is to pretend it is JS:

	Asset_Loader\autoenqueue(
		get_stylesheet_directory() . '/build/asset-manifest.json',
		'style.js', // Note: no JS file will exist.
		[
			'scripts' => [],
			'handle'  => 'mytheme-base-style',
		]
	);

This is confusing and should not be necessary.

[kadamwhite]

As of #22, this should be possible now with

Asset_Loader\register_asset(
    $manifest_file_system_path,
    'asset.css', // May not exist in DevServer context
    [ ... ]
);

Needs documentation but so does everything else in this lib. Closing out this issue, for now!