Values returned from editor.config should not alter config store

[jodator]

It happens to me from time to time and it probably will happen to devs outside the team.

editor.config.set( 'foo.bar', 'bar' );
const foo = editor.config.get( 'foo' );

foo.bar = 'baz';

console.log( editor.config.get( 'foo.bar' ) );
// will log 'baz' not 'bar' :/

I think that values returned from config should be cloned (at least for Objects & Arrays).

This is already handled by every plugin but I tend to forget about that - it's a side effect for me:

• https://github.com/ckeditor/ckeditor5-heading/blob/master/src/utils.js#L26
• https://github.com/ckeditor/ckeditor5-font/blob/fd0c316d2e08baf4a006fc097f9cedeae6d5b723/src/fontsize/fontsizeui.js#L99