Parser fails to recognized single character symbol names

Parser failed with the following VCD line:
```$var wire 1 - D [3] $end```

Reason was that VCDScanner.l contained the following:
```SCOPE_IDENTIFIER    [a-zA-Z_][a-zA-Z_0-9]+```
Changing this to:
```SCOPE_IDENTIFIER    [a-zA-Z_][a-zA-Z_0-9]*```
Fixed the issue.
