Support separating digits with _ as in 1_000 and 100_000.42

```erlang
f()= # _ separating digits
    1_000
    1_23_777
    1_00#9_999_957
    10.000_001E-2_345
```

![](https://raw.githubusercontent.com/fenollp/kju/0139e51fab36ad187f36673b91677d67f1638e35/grammar/test/47.png)