Leverage the new script API strategy feature to defer front end scripts

[adamsilverstein]

This PR adds the defer attribute to the front end script js/rank-math.js enqueued in includes/frontend/class-frontend.php.

Context

WordPress introduced a new strategy feature in the Script API in version 6.3. See this post for more details. Even though js/rank-math.js is only enqueued when the wp-admin bar is shown, it still seems helpful to leverage defer and switch away from the boolean in_footer setting.

Note that this PR includes a version check and slightly different behavior for 6.3 vs. 6.4 where a potential regression is fixed.

Summary

• Detects WordPress version, using new approach when available and falling back to existing approach
• Keeps script in footer for 6.3 while moving to header in 6.4; see https://core.trac.wordpress.org/ticket/59599 for details on why.

Test instructions

• Verify that the script tag includes the defer attribute before and at WordPress 6.3.
• With WordPress 6.3 the script tag will print in the footer and also contain data-wp-strategy="defer indicating the API has added the attribute.
• With WordPress 6.4 the tag will print in the header.