Rename grammars suffix from g4 to g5

[ericvergnaud]

Since we'll be evolving grammar syntax, there is a need to prevent false bugs in antlr4, caused by antlr5 syntax unsupported in antlr4. This is easy to achieve by changing the grammar file extension from g5 to g5.

[KvanTTT]

I suggest postponing this change unless we introduce breaking changes. It would be great if we can parse old grammars without additional effort (there are a lot of grammars in grammars-v4 repository). Moreover some features are back-compatible, for instance, Token value comparators doesn't break compatibility.

[ericvergnaud]

Does antlr4 support the proposed syntax get=ID=='get', without curly braces ?

[KvanTTT]

ANTLR 4 doesn't support. But if support of this feature is introduced, the old grammars also will be still supported without changes.