Add attranslate to Internationalization

[fkirc]

Description

I felt that Flutter is still lacking some tool-support for translations, which is why I wrote attranslate: https://github.com/fkirc/attranslate
Note that this is not a replacement for tools like flutter_translate, but a complementary tool.
It runs only on the developer's machine or CI-workflows (without any impact on running apps).

---

Checklist

• I read How to contribute
• I edited the SOURCE.md file only

---

Feel free to add this badge to your repository after it's accepted to awesome-flutter.

<a href="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/Solido/awesome-flutter">
   <img alt="Awesome Flutter" src="https://img.shields.io/badge/Awesome-Flutter-blue.svg?longCache=true&style=flat-square" />
</a>

[fkirc]

@Solido are there any specific criteria for new tools, perhaps I should close this PR until those criteria are met?

[Solido]

I monitor many metrics but community adoption is the important one since we curate not list all resources.
You can keep it open and ping back if you reach 100 stars, Thanks @fkirc

[fkirc]

100 stars have been reached.
Of course, attranslate is not the first tool that synchronizes translation files, but there aren't that many free open source tools that synchronize both ARB and JSON files.

[Solido]

I try to curate the list and looks like your project just reach the list
Thanks for PR @fkirc

[fkirc]

Thanks for merging, I think that we have now established a new "baseline" for translation synchronization.
So if another Flutter user sets out to create a replacement for attranslate, I has to be at least as good as the current tools.