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]

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Co-authored-by: devansh016 <devansh2002@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[westonruter]

Thank you for the PR. See feedback in #1309 (comment)