Fix ALTER TABLE SETTINGS being parsed per clause

[Lance726]

SETTINGS on ALTER TABLE is a query-level clause: ClickHouse accepts it once, after the last alter clause. It was parsed per clause instead, and implemented by only four of them — ADD COLUMN, ADD INDEX, DETACH PARTITION and DROP PARTITION — so most statements ClickHouse accepts failed to parse:

$ clickhouse-sql-parser -format "ALTER TABLE t MODIFY COLUMN a Int64 SETTINGS alter_sync = 2"
parse statements error: line 1:37 <EOF> or ';' was expected, but got: "SETTINGS"
ALTER TABLE t MODIFY COLUMN a Int64 SETTINGS alter_sync = 2
                                    ^^^^^^^^

Because SETTINGS may only follow the last clause, and any clause can be last, whether a statement parsed came down to which clause happened to be written last. The two statements below are equivalent and both accepted by ClickHouse, but only the second one parsed:

$ clickhouse-sql-parser -format "ALTER TABLE t ADD COLUMN c Int, DROP COLUMN b SETTINGS alter_sync = 2"
parse statements error: line 1:47 <EOF> or ';' was expected, but got: "SETTINGS"
ALTER TABLE t ADD COLUMN c Int, DROP COLUMN b SETTINGS alter_sync = 2
                                              ^^^^^^^^

$ clickhouse-sql-parser -format "ALTER TABLE t DROP COLUMN b, ADD COLUMN c Int SETTINGS alter_sync = 2"
ALTER TABLE t DROP COLUMN b, ADD COLUMN c Int SETTINGS alter_sync=2

SETTINGS is now parsed once in parseAlterTable, after the clause list, so it no longer matters which clause comes last.

Sweeping all 24 alter clauses with a trailing SETTINGS against clickhouse-local 26.8.2.7: 19 forms that ClickHouse accepts now parse and previously did not, 4 already worked and still do, 0 regressions. Mid-statement SETTINGS stays rejected, as ClickHouse rejects it. COMMENT COLUMN still fails, but for an unrelated reason — COMMENT is missing from the clause dispatch altogether, with or without SETTINGS.

Breaking AST change: Settings moves off those four clauses onto AlterTable.Settings. Formatted SQL is unchanged — only the output/ goldens move, not format/.