Media: Introduce `wp_img_tag_add_width_and_height_attr()` to add dimension attributes to images.

Following up on [48170], this changeset moves the new logic to add missing `img` dimension attributes into a separate function that is run first within `wp_filter_content_tags()`. It also adds a utility function `wp_image_src_get_dimensions()` with logic reused from `wp_image_add_srcset_and_sizes()`, and it ensures that `width` and `height` attributes only get added if both of the attributes are missing on the original `img` tag.

This changeset furthermore improves test coverage and separates tests for the different aspects of `img` tag modification.

Props azaozz.
Fixes #50367. See #44427.

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git-svn-id: https://core.svn.wordpress.org/trunk@48006 1a063a9b-81f0-0310-95a4-ce76da25c4cd