TODO: Add `$_['asset']` API

[taufik-nurrohman]

Just like $_['alert'] to pre-create Alert::{$type}() invocation, we likely will need $_['asset'] property too, to manage the control panel assets, so that we don’t have to check if we are in a valid control panel route before invoking any Asset::{$type}() on every panel-related extensions.

The default value should be the front-end asset data converted into array, but with skip property that has been set to true:

$_['asset'] = [
    '.\lot\layout\asset\index.css' => [
        'skip' => true,
        'stack' => 20
    ],
    // ...
];

// If `link` or `url` or `path` property exists, then
// the array key will become an `id` attribute, unless
// a `2['id']` or `id` property exists in the asset item.
$_['asset'] = [
    'asset-0' => [
        'path' => '.\lot\layout\css\index.css',
        'stack' => 20
    ],
    'asset-1' => [
        '2' => [
            'class' => 'custom-class',
            'id' => 'custom-id'
        ],
        'path' => '.\lot\layout\css\index.css',
        'stack' => 20
    ],
    // ...
];

// Example embedded `<style>` tag
$_['asset']['style'] = [
    'foo-bar' => [
        'content' => ':root{--width:600px}',
        'skip' => true,
        'stack' => 20
    ],
    // ...
];

So that any front-end assets will not be loaded into the page. Developers may force the excluded front-end assets to be included in the control panel by removing the skip property or by simply set the skip property value to false:

// Include front-end asset to the control panel
$_['asset']['.\lot\layout\asset\css\index.css']['skip'] = false;
// Make sure to load the asset in the last
$_['asset']['.\lot\layout\asset\css\index.css']['stack'] = 9999;

This is useful for a theme developer who want to make both front-end and back-end appearance to look exactly the same.