[new-parser] DRLParser.g4 and JavaParser.g4 unification

[tkobayas]

Parent issue

• Experiment: New DRL parser #5678

Issue

#5793 (comment)
We are adding new rules in DRLParser.g4 which extends JavaParser.g4 rules, mostly because we need to accept DRL specific lexer tokens. So we have some duplication between those g4's.

At some point in time, review the duplication and consider refactoring. (e.g. fully copy the content of JavaParser.g4 into DRLParser.g4 and modify rules in the single file.)

[tkobayas]

Not high priority