Consolidated the ANTLR4 grammar (#3815)

* Consolidated the ANTLR4 grammar

- Consistent formatting (4 spaces indentation).
- Added `elementOptions` after predicates, which is supported by ANTLR4.
- Renamed `terminal` to `terminalDef` as it conflicts otherwise with the `visitTerminal` standard rule.

* Also allow target language blocks for predicate options.

Author: mike-lischke