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;
...
```

D only supports
```d
version (a) decl
version (a) decl else decl
version (a) decl else: decls
version (a): decls
```