Conditionally load lazysizes if browser doesn't support loading attribute

[jacobarriola]

WP 5.5 now ships with image lazy loading via the native loading attribute.

We should only load @lazysizes if a browser doesn't support this feature.

See: https://web.dev/native-lazy-loading/#what-if-i'm-already-using-a-third-party-library-or-a-script-to-lazy-load-images