Add import/export functionality

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• Closes Add export feature #23

50% 15% written by copilot, wish us all luck

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Issues:

• Defaults are exported too, but shouldn't (the module does not save them into storage at all)
• Importing works, but the form is not updated, so you must manually reload the options page or else you override the import

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Defaults are exported too, but shouldn't (the module does not save them into storage at all)

If I do this, I have two options on import:

• discard options that are stored but not present in the JSON
• not reset missing options to the default value

Overall I think it would be a confusing output, so nah.

Importing works, but the form is not updated, so you must manually reload the options page or else you override the import

Tested in RGH, it works now

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Party-pooping safari as always. Bug known since 2020

https://bugs.webkit.org/show_bug.cgi?id=167341#c55