Refactor remove line separator to method in the rich-text package.

[SergioEstevao]

Description

This PR refactor the logic for removing a line separator inside a list to use a function on the rich text library.

This allows both the native and the web part to share the same code and simplifies the code on the rich text editor component.

How has this been tested?

Screenshots

Types of changes

Refactoring of method.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[etoledom]

Nice step to share more code between mobile and web RichText!

It works well on mobile ✅ But let's wait for @aduth or @ellatrix 's opinion too (or someone else's?), since this also modifies web side files.

A small detail: I think there's a typo in the docs comments. I added it as a code comment.

[aduth]

I can review, but @ellatrix would be in the best position to judge whether this change is a sensible addition to the rich-text API.

[ellatrix]

Sounds good. Good that it's marked unstable.