feat: add spellCheck prop for content editable element

[Dari4sho]

• adds spellCheck as discussed here: https://github.com/mdx-editor/editor/discussions/351

[Dari4sho]

I didn't dive deeper into the library, so I hope it makes sense and I didn't miss something. As mentioned in the discussion, it works fine for me.

[github-actions[bot]]

:tada: This PR is included in version 3.18.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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