Refactor ie11.filter.warnings.es6.js to simpler structure & other improvements

Problem/Motivation

In #3194084: Support functionality equivalent to ckeditor_stylesheets, several improvements to ie11.filter.warnings.es6.js were identified that would provide the same functionality with simpler code. However, those improvements were out of scope so they'll happen here instead.

Steps to reproduce

Proposed resolution

Reviewing the changes in the MR is likely easier than converting the solution to prose. Here's roughly it

• Event listener isn't needed, only MutationObserver
• MutationObserver callbacks can be nested, easier to read.
• Functionality depending on the presence of an error class should use a data attribute instead since the class is technically supposed to impact presentation only.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet