antlr/grammars-v4 ? reverse-engineered prompt

Add a new ANTLR v4 grammar for a language I care about, and make it fit the same style as the rest of this repo.

Keep it clean and action free, with separate lexer and parser rules, a few realistic example inputs, and anything needed so it can be tested the same way the other grammars are. Please follow the folder naming convention used here and make sure it works with the existing repo checks. If you need to look up the latest language syntax online, feel free to do that.