KSES: Allow filter property to accept a URL in safecss_filter_attr().

CSS filters can accept `url()` as a reference to an SVG filter element:
{{{
filter: url( file.svg#filter-element-id );
}}}
This commit allows for that syntax to be used in inline CSS.

Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/48281 #48281 Duotone: Use the style engine to generate CSS for Duotone]

References:
* [https://developer.mozilla.org/en-US/docs/Web/CSS/filter MDN Web Docs: filter()]
* [https://developer.mozilla.org/en-US/docs/Web/CSS/url MDN Web Docs: url()]

Follow-up to [44136], [52049].

Props scruffian, jeryj, ironprogrammer, azaozz, hellofromTonya, SergeyBiryukov.
Fixes #57780.
Built from https://develop.svn.wordpress.org/trunk@55564


git-svn-id: http://core.svn.wordpress.org/trunk@55076 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.3-alpha-55563';
+$wp_version = '6.3-alpha-55564';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.