Media: Allow override of PDF setup for multiple pages or DPI.

After [39187], WordPress started loading only the first page of a PDF.

This is appropriate for performance, but made it impossible to
write plugins that read other pages without overriding `load()`.

Introduces `WP_Image_Editor_Imagick->pdf_setup()`, to allow an override
to change WordPress' rendering DPI defaults or which pages are loaded.

Fixes #38832. See #38522, #31050.
Props markoheijnen, joemcgill, mikeschroder.
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git-svn-id: https://core.svn.wordpress.org/trunk@39243 1a063a9b-81f0-0310-95a4-ce76da25c4cd