Fix off by one error for images (and probably other atomic nodes)

[Nantris]

Changes Overview

Fixes NodePos miscalculation for images

Implementation Approach

Reverts the spirit, but not letter, of the approach that was used before the regression occurred: https://github.com/ueberdosis/tiptap/pull/5038/files#diff-deab15d8918868fc71b9ad8ee3afa808f6b4be07c48e915fd0aed512493fd90cL139

Testing Done

TESTING WAS NOT DONE

Verification Steps

Additional Notes

@masylum your help confirming this PR works as intended would be greatly appreciated

Checklist

• I have created a changeset for this PR if necessary.
• My changes do not break the library.
• I have added tests where applicable.
• I have followed the project guidelines.
• I have fixed any lint issues.

Related Issues

#5314

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[Nantris]

@svenadlung could you take a look at this? I'm not sure if there was a good reason the +1 was removed to begin with or if it was just an oversight. (https://github.com/ueberdosis/tiptap/pull/5038/files#diff-deab15d8918868fc71b9ad8ee3afa808f6b4be07c48e915fd0aed512493fd90cL139)

I'm also unsure whether comparing based on isBlock or isAtom is better (if either one is.)