Add support for flutter/dart "arb" file format

I apologize for mostly copy-pasting this from a KDE bug report I just created.

It looks like google's new language/framework uses an "arb" file format in place of po files.

I am not too familiar with it, but they look like plain json files, and would like to be able to use a dedicated application to edit them. I'd be happy with a barebones implementation for now, although the file format doesn't seem too complex to support.

The specification is available over there

Here is a tutorial with the expected workflow, a possible application, and a few syntax examples.

As an example, the file I attempted to translate is this one, I hope it makes for a good example of real-world usage. The german translation sits in the same folder, as well as instruction in that repository's readme.

A new mime type should probably be created as well, but that can be done independently.