Fix incorrect behavior of `runtime.getURL`

[vitonsky]

Now browser.runtime.getURL returns path from first argument instead of path + extension URL prefix.

I suggest changes to fix this behavior. Here we use a global object to configure extension URL

[clarkbw]

Looks good so far, can you run npm run build and commit that output?

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (f3b766b) 99.50% compared to head (a71d1b6) 99.50%.

Additional details and impacted files

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