Experiment: New DRL parser : Better implementation switch

[tkobayas]

Parent issue: #5678

#5677 (comment)

@mariofusco @yurloc Do you think it's fine to call the new parser DRL10? This doesn't introduce a new syntax, but we need a clear separation. So I use LanguageLevelOption. Feel free to share your thoughts.

Temporarily, yes. But it should be removed in the final step in my opinion.

1. The language level stays the same. We're just changing an implementation detail that deals with how the source code in DRL6 language is being parsed.
2. Is the language level option a public API? If yes, it's one more reason not to introduce DRL10. Implementation details should not proliferate to the public API.

Finally, we would like to use a better way for the implementation switch

[tkobayas]

Done by #5693

Now we use system property switch drools.drl.antlr4.parser.enabled (true by default). No longer use DRL10.