Add caching support

[sebholstein]

Prevent to download the same google fonts URL twice inside a single build run

[kerns]

Download @ build, ...I think this must be what next.js does?

[sebholstein]

@kerns yeah but I asked the astro team if there's any caching system built in, but there isn't atm. So ideally it wouldn't do the download for every build (when the assets are already in the cache).