fix: fix support for thunderbird v127

Turns out Manifest v3 support for Thunderbird is still experimental, and so we probably shouldn't be using it here yet. This was causing unstable API changes to break the extension for certain versions of Thunderbird.

We're now using Manifest v2 instead, which gives us back a stable interface. This is not a breaking change, so the extension supports v127, and still works as intended on v115.

More context: https://matrix.to/#/!IfvTkjNTPAqwpygthx:mozilla.org/$fEeT2S-WTadqmUfr4wcrYOLxo9fpAs7Hs4atSNMui-s?via=mozilla.org&via=matrix.org&via=one.ems.host

Chores

• Updates dev dependencies.
• Fixes another bug that was preventing saving settings when the user pressed to use the recommended instance.

Related

• Fixes #4 (closed)