Update networkUtils.js to check saveData mode

[rafaelstz]

This PR is adding data saver verification before prefetching images.

Data Savers allow users to signal to a site if they would prefer lighter data-usage where possible. PWA Studio respects Save-Data preferences where navigator.connection.saveData is supported by the browser.

[pwa-studio-bot]

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:no_entry_sign:	Unit tests in the following files did _not_ pass 😔. All tests must pass before this PR can be merged packages/peregrine/lib/hooks/useCustomerWishlistSkus/__tests__/useCustomerWishlistSkus.spec.js
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:no_entry_sign:	Missing "Verification Steps" section. Please add it back, with detail.
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:no_entry_sign:	The following file(s) were not formatted with **prettier**. Make sure to execute `yarn run prettier` locally prior to committing. ``` packages/venia-concept/src/ServiceWorker/Utilities/networkUtils.js ```

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