Switch to manifest v3

[rugk]

See TinyWebEx/common#14

[rugk]

Gotta such an error while uploading v2.0:

Use "browser_specific_settings" instead of "applications".

Warnung: The "applications" property in the manifest is deprecated and will no longer be accepted in Manifest Version 3 and above.
manifest.json

[Croydon]

Also required: TinyWebEx/CommonCss#4