Customize: Refactor logic for updating `custom_css` posts by introducing...

Customize: Refactor logic for updating `custom_css` posts by introducing `wp_update_custom_css_post()` function and renaming update filter.

* Moves logic from `WP_Customize_Custom_CSS_Setting::update()` into a re-usable `wp_update_custom_css_post()` function, useful for future REST API endpoint, WP-CLI command, or plugin migrations.
* Renames `customize_update_custom_css_post_content_args` filter to `update_custom_css_data` and improves the naming of the parameters. Instead of passing `post_content` and `post_content_filtered` the filtered array now contains `css` and `preprocessed` respectively. 
* The second context param for the `update_custom_css_data` filter is now an array of the original args passed to `wp_update_custom_css_post()` and there is now no more `$setting` arg since it isn't necessarily being called in the customizer context.

Props westonruter, georgestephanis.
See #35395.
Fixes #38672.

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git-svn-id: https://core.svn.wordpress.org/trunk@39290 1a063a9b-81f0-0310-95a4-ce76da25c4cd