Add export feature

[fregante]

From: refined-github/refined-github#2120

This is probably already feasible via:

copy(JSON.stringify(await options.getAll()))

options.setAll(JSON.parse(paste()))

However it needs to be easier to use or implement. Either:

• add user-facing globals window.webextOptionsSync.export/import() so users can save the config without adding further UI to extensions, or
• add options.export/import() methods that save to/from files, so this feature can be implemented as two buttons (i.e. via <a download> and <input type=file>)

[fregante]

Users can already run these, however accessing the right developer tools can be tricky. Also these instructions only work if the extension loads webextensions-polyfill

Export

1. Open the extension’s options
2. Open the developer tools for the extension’s iframe only
3. Run this

   copy('browser.storage.sync.set(' + JSON.stringify(await browser.storage.sync.get('options')) + ')')

   It will copy a full pastable command to import the options

Import

1. Open the extension’s options
2. Open the developer tools for the extension’s iframe only
3. Paste what you copied and run it