Making Editor Lite extensible

What does this MR do?

Introduces the extensibility nature to Editor Lite. The typical scenario would look like this:

import Editor from '~/editor/editor_lite';
import MarkdownExt from '~/editor/markdown.ext';

[...]

const editor = new Editor();
editor.use(MarkdownExt);

This also supports extension with multiple extensions at the same time. For more information, please check the spec in this MR.

Why?

This would allow keeping the core of the Editor Lite lean and maintainable while extending it only when needed. Also would allow fetching extensions on-demand based on different conditions.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

References #223180 (closed)