Set up GitLab CI, fix reported errors (phpcs, phpstan)

I'm going to work on this and will return tomorrow with the results.

Add basic .gitlab-ci.yml and phpstan.neon (for phpstan) files.

Added fixed for phpstan and phpstan (max PHP version), left comments in MR!22.

Added phpcs, cspell and eslint fixes. Pipeline is green, MR!22 is ready for review.

Take back to work to check why phpstan job doesn't detect deprecations and fix deprecation

------ -----------------------------------------------------------
  Line   src/Form/MediaBulkUploadForm.php
 ------ -----------------------------------------------------------
  450    Call to deprecated function watchdog_exception():
         in drupal:10.1.0 and is removed from drupal:11.0.0. Use
           Use \Drupal\Core\Utility\Error::logException() instead.
 ------ -----------------------------------------------------------

The deprecated code was ignored because of // @phpstan-ignore-next-line.

src/Form/MediaBulkUploadForm.php

  public function batchOperation($id, array $operation_details, array &$context) {
...

    catch (\Exception $e) {
      if (method_exists(Error::class, 'logException')) {
        Error::logException($this->getLogger('media_bulk_upload'), $e);
      }
      else {
        // @phpstan-ignore-next-line
        watchdog_exception('media_bulk_upload', $e);
      }
    }
  }

I removed deprecated code since Media Bulk Upload 3.0.x-dev version is compatible with Drupal ^10.3 || ^11 and Error::logException is present for sure.

Additionally I extended .gitlab-ci.yml to run checks for previous major (in current case Drupal 10.4.2), previous minor (in current case Drupal 11.0.8) and next minor (in current case Drupal 11.x-dev 67cb9b5) versions.

Put back to Needs review.