Support `else:` in conditional declarations

```d
version (A) {
	void foo() {}
} else:
void bar() {}
void baz() {}
```
Else branches in conditional declarations can have multiple declarations. Libdparse cannot parse `else:`, and the AST node of `ConditionalDeclaration` is defined as
```d
...
Declaration[] trueDeclarations;
Declaration falseDeclaration;
...
```

So I fixed the parser, and changed that node definition to:
```d
...
Declaration[] trueDeclarations;
Declaration[] falseDeclarations;
...
```

There also needs to be a rule here for
```
version (a):
  dec;
  dec;
else:
  dec;
  dec;
```