code: public function execute()
      {
          $result = $this->resultJson->create();
          try {
              $read = $this->filesystem->getDirectoryRead(DirectoryList::VAR_DIR);
              $snippetPath = $read->getRelativePath('vcl_snippets_custom');
  
              $customSnippets = $read->read($snippetPath);
  
              if (!$customSnippets) {
                  return $result->setData([
                      'status'    => false,
                      'msg'       => 'No snippets found.'
                  ]);
              }
              $snippets = [];
              foreach ($customSnippets as $snippet) {
                  $snippets[] = explode('/', $snippet)[1];
              }
  
              return $result->setData([
                  'status'    => true,
                  'snippets'  => $snippets
              ]);
          } catch (\Exception $e) {
              return $result->setData([
                  'status'    => false,
                  'msg'       => $e->getMessage()
              ]);
          }
      }
docs: Get custom snippets
  
  @return $this|\Magento\Framework\App\ResponseInterface|\Magento\Framework\Controller\ResultInterface