In the module's README as well as the sub-module READMEs there is the following description:
To use an icon through a custom render array is pretty easy as well.
Just build a render array like this:
```php
$icon = [
'#type' => 'icon',
'#icon_set' => 'icon_set_configuration_entity_id',
'#icon_name' => 'icon_name',
];
```
Or you could use a combination of the configuration entity id with the icon
name like this:
```php
$icon = [
'#type' => 'icon',
'#icon_id' => 'icon_set_configuration_entity_id:icon_name',
];
```
Yet the second example does not work, nor is the text '#icon_id' present anywhere in the module's code.
Attempt to add a render array of an icon using the '#icon_id' syntax.
Add support for utilizing '#icon_id' as described.
Modified Icon::preRenderIcon() to support using '#icon_id' as described.