Fix pasting code in CSS editor

[NiedziolkaMichal]

This PR fixes two issues with pasting code into CSS examples(like matrix):

• When caret is placed inside highlighted syntax, error is thrown(Uncaught TypeError: Cannot read properties of null (reading 'textContent'))
• When user selects text that he wishes to replace, that selection is ignored and new content is always added at the end.

Now editor will use native copy-paste mechanism of the contenteditable elements, we just use Prism.highlightElement(codeElement); afterwards.

There is still one issue regarding this feature. Prism always places caret at the very beginning of highlighted content, which is unexpected to the user. Maybe this can be resolved by using newer version of the library.

@caugner Could you review it please?

[caugner]

@NiedziolkaMichal Would you be able to provide two screen videos, one from before (with the issues you describe), and another one after (with the issues fixed by your PR)?

[NiedziolkaMichal]

Before:

bandicam.2022-05-05.21-02-15-646.mp4

After:

bandicam.2022-05-05.21-07-11-692.mp4

Yellow X is shown before there are too many values and PR #763 is installed on my localhost.

I think MP4 needs to be downloaded for you to be able to see it. For me clicking PLAY button doesn't show any content. I can upload it somewhere else too.

[caugner]

What happens if you paste formatted text that you copied from another website or a Word document? Does Prism handle that correctly?

[NiedziolkaMichal]

It works well. There is a very short glimpse of styled content being placed, immediately replaced by formatted code replaced by Prism.

[NiedziolkaMichal]

Thanks for review. That bug was really annoying, while working on new examples.

[wbamberg]

So much better!