fix(pretty-print): round-trip ALTER TABLE identity, collation and deferral clauses

[psteinroe]

Fix four formatter bugs in ALTER TABLE commands and table constraints where the output either failed to re-parse or silently changed the statement. All seven affected statements from the Postgres 15–17 regression suite (#824) now round-trip at widths 80 and 100.

Identity column options

ALTER COLUMN ... SET GENERATED ... SET INCREMENT ... was emitted with a single leading SET, which doesn't parse, and a bare RESTART became SET RESTART, which Postgres rejects. Each option now gets its own SET, except RESTART, which never takes one. When the options don't fit on one line they break one per line, indented under the column.

-- before
alter column a set generated by default increment by 2 start with 100 restart;
-- after
alter column a
  set generated by default
  set increment by 2
  set start with 100
  restart;

Dropped clauses

• ALTER COLUMN ... TYPE ... COLLATE now keeps its collation, placed before USING as the grammar requires.
• ALTER TYPE ... ADD ATTRIBUTE ... CASCADE now keeps CASCADE.
• EXCLUDE constraints now keep DEFERRABLE / INITIALLY DEFERRED. The deferral clause is now one helper shared by PRIMARY KEY, UNIQUE and EXCLUDE. No existing snapshot changes.

Refs #824