Add support for flutter Application Resource Bundle file ".arb"

[benoit-rolandeau-act]

The Application Resource Bundle file (.arb extension) are used by flutter to generate localisation files. The format of the file is based on JSON. See here: https://github.com/google/app-resource-bundle

Therefore, I think it will be right to linked ".arb" files with the class: reuse.comment.UncommentableCommentStyle (as it's done with JSON).