feat: add ability to set custom icon to editor panel blocks

[hdziahonskaya]

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[m-Ryan]

#408
Please check this PR, by setting the class name for the block item, you can redefine the corresponding icon @hdziahonskaya

[hdziahonskaya]

@m-Ryan, I see that icons could be redefined by class names. But won't the enhancement in PR provide greater flexibility and control over block visuals and simplifies integration and customization for users of package?