Option to mark terms as deprecated

[RobertOdrowaz]

Having an option to mark some terms as deprecated before removing them from poeditor while they are stil in use in the app would be useful. Poeditor allows assigning tags to terms so propagating tag "deprecated" to @depreacted in the code would probably be best.

This feature might not be simple or even possible because (in this form):

1. I'm not sure if those tags are event available though poeditor api
2. Probably requires editing generated .dart files since I don't think there is a way to mark term as deprecated in the .arb format

[Albert221]

POEditor's JSON export doesn't give information about tags:

POEditor	Export
	{ "term": "testPlural", "definition": { "one": "One {count}", "other": "Many {count}s" }, "context": "", "term_plural": "plural", "reference": "", "comment": "" },

It is possible to fetch this tags info from POE API: https://poeditor.com/docs/api#terms_list

And yes, your 2. point is correct. This will require modifying the flutter gen-l10n output Dart files. So we either make poe2arb also run the flutter gen-l10n command or create a separate command that serves solely for spicing the Dart files with (for now) @deprecateds.