replace rollup-plugin-off-main-thread in workbox-build to fix deprecation warnings

[Trickfilm400]

Library Affected:
workbox-build

Browser & Platform:
"all browsers"

Issue or Feature Request Description:

Hi, when installing workbox-build you get a deprecation warning of packages inside the dependencies; notably @surma/rollup-plugin-off-main-thread. As this project seems unmaintained, I've created a fork and patched the warnings and some other changes.

So the request would be to change @surma/rollup-plugin-off-main-thread to @trickfilm400/rollup-plugin-off-main-thread.