Rename grammars suffix from g4 to g5

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.

The issue requests changing the grammar-file suffix, but its title says g4 to g5 while the body says g5 to g5. No files or tests are named, so first clarify the intended direction and locate where grammar filenames are handled. Done means the chosen suffix is applied consistently and prevents antlr4 from accepting incompatible antlr5 syntax.

Written by the indexing model from the issue text.