Cache Playground assets to enable offline support

[bgrgicak]

Motivation for the change, related issues

The promise of Playground is WordPress anywhere.
Playground can run in the browser without a server, but it still requires the internet on every load.

This PR adds asset caching of Playground assets and ensures they are available when offline.

Implementation details

The PHP-wasm service worker will now start the cache when initialized and clean previous versions if they exist.

On each unscoped fetch request, it will attempt to load data from the cache by using the cachedFetch method.

The WorkerCache class uses the browser cache to preload data when the service worker is installed. It also caches any other valid unscoped fetch request to speed up reloads of Playground. The isValidHostname method checks if the request should be cached.

To ensure remote assets are available while offline they are loaded during boot.
If Playground is online they will still be loaded in the background to reduce the number of server requests.

Testing Instructions (or ideally a Blueprint)

• Follow the Test offline support instructions
• After loading Playground in your browser turn off the server
• Refresh the page
• Confirm that Playground loaded
• Browse the site, go to WP-admin, make changes...
• It should all work as usual
• Change the PHP or WP version and confirm it works

Note: In Chrome the offline checkbox sometimes stops working. When this happens Playground will load without remote assets and admin styles (including the admin bar) will look broken. If it happens you can confirm it by running window.navigator.onLine. To resolve it uncheck and check the Offline checkbox and reload.

TODO

• We currently cache all PHP and WP versions on install. This is a lot of data, so we should decide if it's worth doing it, or if we should load only the current PHP and WP versions.
• Precache runs on install and doesn't block Playground from loading, but that also means it's not clear when all cache files are downloaded.

These issues will be resolved by the redesign #1561, but we should still decide what's the best solution in the meantime.

[bgrgicak]

Playground can now load from the cache when there is no server.

There is still an issue on the first load where some static assets aren't pulled from cache.
Also, assets are cached only on the second load.

Other TODOs:

• Store cache with a version prefix
• Flush cache when version changes

[bgrgicak]

#1541 adds a cache version to the Web service worker.
Once this is merged I can update this PR with cache busting and cleaning.

[bgrgicak]

@adamziel this will download all PHP and WP versions into cache.

Is this ok for v1 or should we limit it to the current WP and PHP version?
If yes, users won't be able to switch versions while offline.

[adamziel]

I would limit this for v1 to avoid adding 200-300mb of extra transfer. No switching is fine for now, we'll find the right way of downloading the missing assets and communicating that to the user as a part od the webapp redesign issue.