Migrate to Manifest Version 3

[DNin01]

If you're not currently aware, Extensity is still on the Manifest V2 platform... https://github.com/sergiokas/Extensity/blob/179da93f5c6240046907ede761f4a1b94fd8fb66/manifest.json#L4 ...but the latest is version 3. Even more important, Google will be discontinuing support for MV2 extensions in 2023, which means this extension will have to start using MV3 soon if we want it to keep going. I hope we can see this extension updating to MV3 sometime soon if the plan is to keep it around.

More information about MV3 and how to update to it is available at Google's Welcome to Manifest V3 - Chrome Developers page.