[Performance] Constants for JavaScript runtime should be folded

https://github.com/antlr/antlr4/issues/3698 but for JavaScript

Benchmark code: https://gist.github.com/KvanTTT/e3b355f7e321fe7f52e11ea1aa0ecbce#file-javascript_constant_folding_benchmark-js

Result (node v14.16.1):

```
no_folding_const_refs_test: 538 ns
no_folding_const_literals_test: 67 ns
folding_test: 70 ns
```

