[mssql] Support DECLARE statements

I am running into a few errors trying to parse MsSql.

The following sql
```sql
DECLARE @Var1 INTEGER;
```

reports the following error:

```
ParserError("Unexpected keyword \"DECLARE\" at the beginning of a statement")
```
This is a pretty normal thing to appear in t-sql scripts from my experience, though I maybe misunderstanding why this might be an error.