Plugin: Copy wp_should_load_separate_core_block_assets function from core

[aristath]

Description

The gutenberg_should_load_separate_block_assets function was ported to core as wp_should_load_separate_core_block_assets. Instead of adding a gutenberg_ function in the plugin and calling wp_ inside it, we can simply do a function_exists check, and add the wp_ function in the plugin if the function doesn't exist.

This will make maintenance easier and will allow for easier porting of future code in core.

How has this been tested?

Tested with a block theme and a classic theme in WP5.8-trunk, everything works as expected.

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).

[gziolo]

It all depends on whether we are planning to apply any changes to the implementation that exists in WordPress core. It contains a filter so we probably can always use it instead.