Add SQL_TXN_* types to TypeScript definition

The odbc object contains transaction isolation level constants that
were not included in the TypeScript definition. They could be accessed
at runtime but the TypeScript compiler would give a compile-time error.

This commit exposes the following constants (all have type `number`):

* `SQL_TXN_READ_UNCOMMITTED`
* `SQL_TXN_READ_COMMITTED`
* `SQL_TXN_REPEATABLE_READ`
* `SQL_TXN_SERIALIZABLE`

Note that the odbc object also has constants with the full word
"transaction" (ex: `SQL_TRANSACTION_READ_UNCOMMITTED`) that have the
same value. It also has constants for various SQL types.

Signed-off-by: BrendanC23 <4711227+BrendanC23@users.noreply.github.com>

Author: BrendanC23

lib/odbc.d.ts

@@ -205,6 +205,11 @@ declare namespace odbc {
 
   function pool(connectionString: string): Promise<Pool>;
   function pool(connectionObject: PoolParameters): Promise<Pool>;
+
+  const SQL_TXN_READ_UNCOMMITTED: number;
+  const SQL_TXN_READ_COMMITTED: number;
+  const SQL_TXN_REPEATABLE_READ: number;
+  const SQL_TXN_SERIALIZABLE: number;
 }
 
 export = odbc;