Fix kses filter for un-prettified filters

[ajlende]

What?

Updates the regex to allow for zero spaces between filter: and url(.

Why?

Fixes #48974

How?

The style engine has an option where it doesn't include spaces if SCRIPT_DEBUG is true.

It was causing custom duotone filters (including presets applied prior to GB 15.2.4 which were treated as custom filters) to be stripped by kses because the lack of spaces wasn't accounted for in the regex.

Testing Instructions

1. Ensure define( 'SCRIPT_DEBUG', false ); is set in wp-config.php
2. Add a block with a custom duotone filter to a post/page. Do not use the presets.
3. View the post/page in the frontend.
4. Duotone filter should be applied.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

Flaky tests detected in df02eba.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4389377819
📝 Reported issues:

• [Flaky Test] should filter results properly #44462 in specs/site-editor/global-styles-sidebar.test.js

[scruffian]

LGTM

[scruffian]

I think we should add tests for this but I'm going to do that in a different PR.

[scruffian]

Here's a unit test for this: #49033