Introduce `has_image_size( $name )`, a utility function that helps avoid loading the `$_wp_additional_image_sizes` global.

Props mordauk.
Fixes #26951.


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Add remove_image_size() and tests for it and has_image_size(), added in [27128].

props mordauk, markoheijnen.
fixes #26768. see #26951.

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Add a filter `wp_insert_attachment_data` in `wp_insert_attachment()`. This is similar to the `wp_insert_post_data` filter in `wp_insert_post()`.

Fixes #20547.


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Various JSHint fixes. see #26446.

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Improve parameter documentation of add_meta_box().

props philiparthurmoore, DrewAPicture.
fixes #26698.

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