custom_module.services.yml

services:
  custom_module.custom_module_twig_extensions:
    arguments: ['@renderer']
    class: Drupal\custom_module\TwigExtension\CustomModuleExtensionAdapter
    tags:
      - { name: twig.extension }

CustomModuleExtensionAdapter.php

namespace Drupal\custom_module\TwigExtension;

use Twig\Extension\AbstractExtension;


/**
 * Collect custom twig functions and filters
 */
class CustomModuleExtensionAdapter extends AbstractExtension
{

    /**
     * CustomModuleExtensionAdapter constructor.
     */
    public function __construct() {
        CustomModuleExtensionLoader::init();
    }

    /**
     * Get all Twig functions.
     */
    public function getFunctions() {
        return CustomModuleExtensionLoader::getFunctions();
    }

    /**
     * Get all Twig filters.
     */
    public function getFilters() {
        return CustomModuleExtensionLoader::getFilters();
    }

}

CustomModuleExtensionLoader.php

namespace Drupal\custom_module\TwigExtension;

/**
 * Load individual extensions
 */
class CustomModuleExtensionLoader
{

    /**
     * Set of Twig functions
     *
     * @var array
     */
    public static $functions = [];

    /**
     * Set of Twig filters
     *
     * @var array
     */
    public static $filters = [];

    /**
     * Calls loader for Twig helpers
     */
    public static function init() {
        if (!self::$functions) {
            static::loadAllFunctions();
        }
        if (!self::$filters) {
            static::loadAllFilters();
        }
    }

    /**
     * Get all Twig functions
     */
    public static function getFunctions() {
        return !empty(self::$functions) ? self::$functions : [];
    }

    /**
     * Get all Twig filters
     */
    public static function getFilters() {
        return !empty(self::$filters) ? self::$filters : [];
    }

    /**
     * Load all Twig functions
     */
    protected static function loadAllFunctions() {
        $fullPath = drupal_get_path('module', 'custom_module') . '/twig-components/functions/';
        if (is_dir($fullPath)) {
            static::load($fullPath . 'add_attributes.function.drupal.php', self::$functions);
            static::load($fullPath . 'svg_icon.function.drupal.php', self::$functions);
        }
    }

    /**
     * Load all Twig filters
     */
    protected static function loadAllFilters() {
        $fullPath = drupal_get_path('module', 'custom_module') . '/twig-components/filters/';
        if (is_dir($fullPath)) {
            static::load($fullPath . 'unique_id.filter.drupal.php', self::$filters);
        }
    }

    /**
     * Load a single Twig dependency
     *
     * @param string $file
     * @param array  $collection
     */
    protected static function load(string $file, array &$collection) {
        $function = NULL;
        if (file_exists($file)) {
            include $file;
            $collection[] = $function;
        }
    }

}