fix: peek button not showing on newer UI

[thenbe]

The button was not consistently showing in github lately (depending on the version of the UI you're getting). I kept the original selectors, but added new ones as a fallback to maintain backwards compatibility. Some related issues still remain (#13), but this is a start.

I had to explicitly install webextension-polyfill, otherwise esbuild fails to build.

Finally, I also added debug logs. They're not necessary for this PR, but will be useful for anyone debugging this in the future.

Tested on firefox 120.0.1 and chrome 120.0.6099.109