fix: add support for pasting plain text into editable void

[alex-vladut]

Description
This is a fix for the issue preventing pasting plain text into editable voids. At the moment one can paste other Slate elements into an editable void, but nothing happens when plain text (e.g. copying a URL from browser's search bar).

Issue
Fixes: #5294

Example

Before:
https://user-images.githubusercontent.com/3620639/219029517-0f7b11e3-78ae-4033-af74-61730103ae04.gif

After:

copy.and.paste.into.editable.voids.mov

Context

A change was introduced recently to support copying and pasting void elements, which seems to have changed the behaviour for the paste action as you can find it here https://github.com/ianstormtaylor/slate/pull/5121/files#diff-1d0c52ec4c5562d96965f789988bd4efd4a6d24b88527918987d06bcc43d15adL1576
It is not fully clear to me if that change was intentional or it was simply added to match the onCopy logic. I performed some testing according to the original PR's purpose and the copy-paste of void elements seems to still work as expected. Let me know if anyone has more insights and there is something that should be checked.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: cde6c52

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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