get_block_wrapper_attributes: Ensures that user-provided attributes override the attributes generated by block supports

[t-hamano]

Note

This PR is the same as #10877. I submitted this new PR because I accidentally closed #10877 and it can no longer be reopened.

Trac ticket: https://core.trac.wordpress.org/ticket/64603

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[westonruter]

I know I suggested $handled_attributes originally, but now that I look at it it seems confusing because a false would seem to indicate that the attribute is not handled. So what about renaming $handled_attributes as $merged_attributes?

[westonruter]

For posterity, #10877 (comment):

In the case of merging style, should special handling be added to ensure that $extra_attributes[ $attribute_name ] ends in a semicolon? The current empty space is only really appropriate for class.