Add DrupalCI step for ensuring that CKEditor 5 build files are build correctly

without looking too deeply into it, seems 100% related to #3238497: What to do with assets build step? and what sally proposed, that drupal ci does the compile step somehow.

ugh, bash… too young for this :p I'll get back to it next week

Manually tested, works great!

Tested:

1. no changes, just this MR:
   

   $ yarn run -s check:ckeditor5
   $ echo $?
   0
   

2. made this change:
   

   diff --git a/core/modules/ckeditor5/js/ckeditor5_plugins/drupalMedia/src/drupalmediametadatarepository.js b/core/modules/ckeditor5/js/ckeditor5_plugins/drupalMedia/src/drupalmediametadatarepository.js
   index fc023b6cb8..ca8c37b0be 100644
   --- a/core/modules/ckeditor5/js/ckeditor5_plugins/drupalMedia/src/drupalmediametadatarepository.js
   +++ b/core/modules/ckeditor5/js/ckeditor5_plugins/drupalMedia/src/drupalmediametadatarepository.js
   @@ -23,7 +23,7 @@ const _fetchMetadata = async (url) => {
        return JSON.parse(await response.text());
      }
    
   -  throw new Error('Fetching media embed metadata from the server failed.');
   +  throw new Error('Fetching media embed metadata from the server failed dramatically.');
    };
    
    /**
   

3. $ yarn run -s check:ckeditor5
   $ echo $?
   1
   

Once the nits are addressed, this is RTBC!

updated the comments

Re-confirming RTBC :)