fix double insert in anchor element decorations

[BitPhinix]

Description
Adjusts the fix in 9957c21 to truly catch all edge-cases.

Right now it doesn't work for decorations since the Editor.isEnd() check will return false for decorations that split text node.

The createTreeWalker approach was the fastest way I could find to get the last dom text node inside the anchor element. Maybe there is a smarter way to do it using e.g. document.evaluate.

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: 3d23718

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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