Fix several issues of identifiers (#147)

This PR fixes most of the remaining issues related to identifiers as
reported in #142 .

```nushell
# values
null
7b
nan
# commands
g++
7z
7mss
truee
```

It's still not perfect, nushell handles special characters like `-, #,
., ()` in identifiers very dynamically.
I think this is the time to finally resort to an external scanner for
further accuracy.