Add filter for enabling of taxonomies

A filter would be great that can be used to enable this plugin (featured images) for specific taxonomies.
Another plugin can then programmatically enable featured image support for specific taxonomies without the need for the user to enable these manually. A filter could be inserted into the `taxonomy_image_plugin_check_taxonomy` function:
https://github.com/Automattic/Taxonomy-Images/blob/0c4bb009820ff6db871f9e5707a7811c6f70878a/taxonomy-images.php#L1091

Edit: Current workaround using the plugin options filter (https://github.com/Automattic/Taxonomy-Images/blob/0c4bb009820ff6db871f9e5707a7811c6f70878a/taxonomy-images.php#L1101):
```php
// @see https://github.com/Automattic/Taxonomy-Images/issues/6
add_filter( 'option_taxonomy_image_plugin_settings', function( $option_value ) {
$ensure_taxonomies = [ 'my_taxonomy_slug', ];

$option_value['taxonomies'] = array_unique(
array_merge(
$option_value['taxonomies'],
$ensure_taxonomies,
),
SORT_REGULAR
);
return $option_value;
} );
````

Read taxonomy-images.php at taxonomy_image_plugin_check_taxonomy around line 1091, then compare the option filter at line 1101 and its documented workaround. Confirm the filter can programmatically enable featured-image support for selected taxonomies, with the existing behavior preserved.

Written by the indexing model from the issue text.