Add linkage attribute parsing support to alias declarations.

Fixes https://github.com/Hackerpilot/Dscanner/issues/66

Note that I've not added support for a lone `extern` as a storage class. Even though DMD currently parses the following, I don't think this declaration makes any sense:

```D
alias extern void function() f;
```

I've **never** seen such code in the wild.

Support was added for:

```D
alias extern(Linkage) void function() f;
```

But support was **not** added for:

```D
alias f = extern(Linkage) void function();
```

This is currently a bug in DMD, so I'm not sure whether to add support or to keep in line with what DMD supports in the latest stable version.