Use VARCHAR(255) for Firebird string default

The Firebird generator was using BLOB SUB_TYPE TEXT as the default type for AsString(). I've changed it to use VARCHAR(255) instead since this is more likely what is actually desired. This is now the same behavior as the Jet, MySQL, Oracle and SqlServer2000 generators.

```
column.AsString() => VARCHAR(255)
column.AsString(100) => VARCHAR(100)
column.AsString(10000000) => BLOB SUB_TYPE TEXT
```