Add workflow to validate translation JSON

[headquarter8302]

This PR adds a GitHub Actions workflow Validate translation files, which will execute jq against added and modified JSON files within the assets/translations directory.

The steps are:

1. Checkout repo
2. Fetch the JSON files from the PR through viewing the diff, and only those that are in assets/translations
   • Also send filenames to Actions output
3. Perform silent validation of JSON files from previous step's output

I've noticed that some updates to the app are only for fixing broken translation files, I thought this workflow should help catch these problems before they're released