Set fetchpriority to low if image is in the initial viewport

[devansh016]

Summary

Set fetchpriority to low if image is in the initial viewport but is not visible

Fixes #1309

Relevant technical choices

if ( ! $is_visible ) {
	$walker->set_attribute( 'fetchpriority', 'low' );
}

[github-actions[bot]]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: devansh016 <[email protected]>
Co-authored-by: westonruter <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.