Add `_next` property to the Select interface type for UNION queries

[aok-solutions]

Describe the bug
When SQL Select statements use a UNION operator, the generated AST has a _next property that contains the ASTs for the other tables being unioned. However this property is not exposed in the Select interface type so the typechecker fails when trying to access those nodes.

Database Engine
Snowflake

To Reproduce
node: 18.20.1
node-sql-parser: 5.3.2

import { Parser } from "node-sql-parser";

const parser = new Parser();
const opts = { database: "Snowflake" };

const query = `
        SELECT user_id, timestamp
        from login.account.password_reset
        union
        select user_id, timestamp
        from "LOGIN"."ACCOUNT"."PASSWORD_UPDATED"
        union
        select user_id, timestamp
        from LOGIN.ACCOUNT.SUCCESSFUL_LOGIN;
`;

const { ast } = parser.parse(query, opts);
const firstUnionTable = ast[0]._next

error TS2339: Property '_next' does not exist on type 'Select'

Expected behavior
should be able to access _next without typescript throwing an error