Duotone: Avoid rendering duplicate stylesheet and SVG

[Mamaduka]

Description

Adds check to avoid rendering duplicate stylesheet and SVG elements when the plugin is used with WP 5.8.

How has this been tested?

1. Create a post.
2. Add an Image block.
3. Select the Duotone option.
4. Save the post.
5. On the front-end, WP shouldn't render duplicated stylesheet and SVG for Duotone.

Screenshots

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[jorgefilipecosta]

This basically uses the wp-code if it is present. It makes future changes in the plugin impossible to do because the core version is going to be used. Could we remove the core filters instead if they are present?

[Mamaduka]

Makes sense. I'll update the code.

This is a pattern I've seen before in Gutenberg core and the only reason I've used it here.

[Mamaduka]

Done ✅

P.S. I pushed a similar fix for Group Block layout support. Do you want me to update logic there as well to use remove_filter?

[ajlende]

Tested locally, and it's working fine for me!