Use _builtin property of classes in navigation link PHP

[spacedmonkey]

Description

Use the _builtin property of the WP_Post_Type and WP_Taxonomy classes instead of hardcoring.

How has this been tested?

Screenshots

Types of changes

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).

[Mamaduka]

Tested and works as expected. I love the improved check.

P.S. I used following snippet to get block variations:

wp.blocks.getBlockType('core/navigation-link').variations;

[gwwar]

Nice improvement, thanks @spacedmonkey !