omit Sql properties absent from TransactionSql in type definition

TransactionSql is missing properties from Sql added in https://github.com/porsager/postgres/blob/32feb259a3c9abffab761bd1758b3168d9e0cebc/src/index.js#L69-L82. This PR updates the type definition of TransactionSql to reflect this. This means typescript will now raise an error if consuming code tries the following: ```ts sql.begin((trx) => { trx.begin((trx2) => { ... }) }) ```