ES5 support could be added into `antlr4ts` via https://github.com/tunnelvisionlabs/antlr4ts/pull/440, but this PR is currently blocked. There are two tracking issues in tunnelvisionlabs/antlr4ts: [#311](https://github.com/tunnelvisionlabs/antlr4ts/issues/311) and [#326](https://github.com/tunnelvisionlabs/antlr4ts/issues/326).