Cache and prefetch shield files

[Duddino]

Abstract

When creating or loading a shield wallet, a bunch of helper scripts are fetched from the server sequentially. This happens one time for each thread.
Even though they're cached, a request is still sent to the server to see if the file changed, this creates a noticeable between creating a wallet and getting to the dashboard, particularly on machines with a high number of cores, and/or with a high network latency.

This PR aims to prefetch the required shield files, before they're actually needed. The idea is that by the time someone has created and backed up their seedphrase, the browser would have already fetched the files.
Additionally, this PR adds the util scripts each worker needs in the sapling cache to avoid contacting the server for each worker.

Testing

• Have a third world connection (or simulate it using the devtools)
• Open MPW and try to create a new wallet or import an existing one, and see if there is any noticeable lag

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[panleone]

nice PR

[panleone]

tACK e6191be

[JSKitty]

Receiving error after simply opening the page, Firefox on Linux.

Uncaught (in promise) TypeError: registration.active is null
    registerWorker native.js:15

[panleone]

tACK b3c1e7e

noticeably faster than the master branch

[JSKitty]

tACK c71fed4

Took a few attempts for my configuration, but undeniably faster loading speed now. Happy this was taken care of. 🙏