Prefix file paths in SW manifest with a slash

[0xd61]

This PR adds a "/" to the file paths in the SW manifest file during compile time.
Fixes #1210

[benmccann]

@antony should we apply the "breaking change" label to this one?

[antony]

@benmccann why do you consider this breaking?

[benmccann]

For folks that already updated their service worker to match the existing paths, this change would break their caching. I think it's rare that folks have fixed their apps because I see broken apps in the wild all the time, so this change would fix the majority of apps and probably only a small minority would need to revert their service workers to match this change. But it's worth calling out in the changelog as a breaking change so folks can check and be aware if they previously updated their service worker to deal with this issue that they'll need to update it again

[lukeed]

Breaking something by fixing it is probably the best kind of break, haha. Would also mean any workaround for anything has to be considered before patching.

[antony]

Seems to work, sorry it took so long to review!