Uncaught TypeError: CKEDITOR.addCss is not a function (CK3)

Problem/Motivation

Encountering a `Uncaught TypeError: CKEDITOR.addCss is not a function` for CKEdtior 3 library versions

Steps to reproduce

Enable Footnotes in `admin/config/content/ckeditor/edit/[format]` .

Note: After dragging the plugin on the editor plugin toolbar, tick the "Add Footnotes" to enable it.

Proposed resolution

Add filter check by determining CKEditor version and use "editor" variable method instead of global CKEDITOR.

Also, cleaned up css string to separate constant.