Add support for preserving text attributes on enter

[msamsel]

Suggested merge commit message (convention)

Feature: Add support for respecting copyOnEnter attribute's parameter. Closes ckeditor/ckeditor5#4562.

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Additional information

Dependent PR:

• Mark alignment attribute with 'copyOnEnter' property ckeditor5-alignment#41
• Mark basic-styles attributes with 'copyOnEnter' property ckeditor5-basic-styles#92
• Mark fonts' attributes with 'copyOnEnter' property ckeditor5-font#48
• Add description for 'copyOnEnter' attribute's property ckeditor5-engine#1741

[coveralls]

Coverage remained the same at 100.0% when pulling 661e550 on t/40 into 6942320 on master.

[jodator]

Small comments to the code. I'll check the behavior in the meantime as well.

[progmancod]

Is it possible to set this same behavior to the heading plugin? I mean, copy the element class.

[Reinmar]

Is it possible to set this same behavior to the heading plugin? I mean, copy the element class.

That's a separate piece (subfeature). Here we're copying text attributes. Copying element attributes would be a separate ticket.

[msamsel]

@Mgsy here @jodator already start a review, however, you can also take a look on functionality part. Maybe there exist some not obvious relations and connections, which you might notice.

[Mgsy]

Steps to reproduce

1. Set the following data: editor.setData("<p>test</p>").
2. Put the caret at the end of "test".
3. Activate bold.
4. Press Shift + Enter.

Current result

The attribute is lost.

GIF

[jodator]

Small updates to the tests, docs and one API usage.

[jodator]

All is good now. You only missed the selection.getAttributeKeys() use instead of custom keys retrieval from selection.getAttributes().

[msamsel]

Yeah, I missed this method somehow 🤦‍♂.