fix(extension): load devtools-background.js as type="module"

[skirtles-code]

Since d8e2801, the file devtools-background.js has been built as an ES module, rather than an IIFE.

I'm not really familiar with how extensions work, or what the intent was behind switching to an ES module, but when I try to install a local copy of the extensions they don't work. Adding type="module" seems to be sufficient to get them working.

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commit: b9a2603