Prevent assets from being added to assets array twice

[borkweb]

When adding assets, an asset would be added to the Assets::$assets array with the slug as the index and with a numeric index. There's no reason for the numeric index, as it just causes extra looping and attempts to enqueue.

Before:

[
  'my-style'  => Asset /* my-style */,
  0           => Asset /* my-style */,
  'my-script' => Asset /* my-script */,
  1           => Asset /* my-script */,
]

After

[
  'my-style'  => Asset /* my-style */,
  'my-script' => Asset /* my-script */,
]

[dpanta94]

What the check here is doing doesn't seem enough to be sure about what we want to achieve. We might pass an invalid array and the error would still be there and it could be even more serious.

It should be instead:

if ( ! is_array( $assets ) {
    $assets = [ $assets ];
}

foreach ( $assets as $asset {
    if ( ! $asset instanceof Asset ) {
        throw
    }
    
    $this->assets[ $asset->get_slug() ] = $asset;
}

return;