Formatting: new filter `safecss_filter_attr_allow_css` on css parts.

Enables developers to determine whether a section of CSS should be allowed or discarded. By default, the value will be false if the part contains \ ( & } = or comments. Returning true allows the CSS part to be included in the output.

Replaces the `safe_style_disallowed_chars` filter introduced in r47891.

Props azaozz.
Fixes #37134.


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git-svn-id: https://core.svn.wordpress.org/trunk@47853 1a063a9b-81f0-0310-95a4-ce76da25c4cd