docs: Updates inject-manifest docs to explicitly mention devOptions

[lukebatchelor]

Just a small docs update to hopefully save someone a lot of pain.

Whilst following the docs for injecting a custom service worker I kept running into the Cannot use import statement outside a module error. After spending a long time on it, I eventually found that it's 100% answered in the docs already, so I'm just making that more clear.

I think this could be a pretty common use case, a user is developing a custom service worker, they're reading the custom service worker docs and they're copying the code samples with imports in them, so hopefully this is useful.

Cheers

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