refactor(media): Refactor month retrieval and add new function for easier caching

[apermo]

This introduces get_media_library_months_with_files() just as described in 63279, this allows an easy and consistent way to create transients in media_library_months_with_files without the need to copy and paste WordPress core code.

Trac ticket: https://core.trac.wordpress.org/ticket/63279#ticket

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[JasonSa19]

LGTM