feat: Add sessions namespace to metadata
feat: Add sessions namespace to metadata

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/sessions#Browser_compatibility

Only `sessions.getRecentlyClosed` and `sessions.restore` are currently
supported by Firefox.

`sessions.getDevices` is included anyway since this method may be added
to Firefox in the future: https://bugzil.la/1308059

brightbytes/webextension-polyfill,brightbytes/webextension-polyfill