Fix S001 implicit-cross-join false negatives (aliases, schema-qual, multi-line)

[Pawansingh3889]

Closes #41.

S001 (implicit-cross-join) already exists, but the detection regex \bFROM\s+(\w+\s*,\s*\w+) only matches when the comma comes immediately after the first word after FROM. Most real-world comma-joins slip through silently.

Reproductions on main (none of these flag):

SELECT * FROM orders o, customers c WHERE o.cust_id = c.id;
SELECT * FROM raw.orders, raw.customers WHERE raw.orders.cust_id = raw.customers.id;
SELECT * FROM o1, o2, o3 WHERE o1.id = o2.id AND o2.id = o3.id;
SELECT *
FROM orders o,
     customers c
WHERE o.cust_id = c.id;

Replaces the regex with a paren-depth-aware scan. From each FROM keyword, walk forward tracking parenthesis depth, stop at WHERE / GROUP BY / ORDER BY / HAVING / LIMIT / FETCH / OFFSET / explicit JOIN (all variants) / UNION / EXCEPT / INTERSECT, and flag the first depth-0 comma. Strings and comments are stripped first so commas inside literals or comments do not trigger.

Two deliberate suppression cases. DELETE FROM is skipped because a single-target DELETE is the normal case in standard SQL and bare DELETE FROM x, y is invalid in Postgres, SQL Server, and MySQL. Comma followed by LATERAL is recognised as a legitimate Snowflake / Postgres lateral join and not flagged, so FROM events e, LATERAL FLATTEN(input => e.tags) and FROM t1, LATERAL (SELECT ...) both pass.

Why fix S001 rather than ship a duplicate W027: the rule already existed and was simply not catching the patterns it claimed to cover. Adding W027 would have left the buggy S001 in place. Closing #41 here.

Tests: 14 new regression cases under TestImplicitCrossJoin covering aliases, schema-qualified names, three-or-more-way joins, multi-line layout, comma-join inside CTE, plus all the suppression cases. Test class size goes from 3 to 17. Full suite: 226 pass and 1 skipped, no regressions.

CHANGELOG entry added under [Unreleased] Fixed.

Commits signed (ED25519).