Misc. webpack changes

[jeffposnick]

R: @Snugug @philipwalton

Fixes #2681, fixes #2674

These are some small, final webpack-related changes I'd like to get in before the final Workbox v6 release.

• Bump the minimum required version of webpack v5 to v5.9.0, as that includes some changes that is needed for Workbox v6.
• Use Error objects when adding to the compilation warnings instead of strings.
• Treat multiple instances of self.__WB_MANIFEST in the swSrc as a compilation error, matching the workbox-build behavior.