Support MERGE statement in PostgreSQL

[emirmrkaljevic]

Closes #986

PostgreSQL has had MERGE since version 15, but the postgresql dialect didn't know about it, so MERGE INTO, WHEN [NOT] MATCHED and the bare INSERT inside a merge action weren't treated as clauses and kept their original case.

Changes:

• Added MERGE INTO, WHEN [NOT] MATCHED [BY SOURCE | BY TARGET] [THEN] and UPDATE SET to the PostgreSQL reserved clauses (BY SOURCE / BY TARGET are PostgreSQL 17 syntax).
• Changed INSERT INTO to INSERT [INTO], since a merge action is just INSERT (cols) VALUES (...) without INTO. This is the same as what transactsql and plsql already do.
• Added MERGE_ACTION (PostgreSQL 17) to the function list, so merge_action() in RETURNING isn't formatted as merge_action ().

Tests: the PostgreSQL suite now runs the shared supportsMergeInto test, plus a PostgreSQL-specific test covering WHEN MATCHED AND ..., DELETE, DO NOTHING, BY SOURCE / BY TARGET and RETURNING merge_action().

One side effect to note: because bare INSERT is now a clause in PostgreSQL, something like GRANT INSERT, UPDATE ON t TO bob breaks before INSERT the same way it already broke before UPDATE. Happy to narrow it down if you'd rather avoid that.

pnpm test, pnpm ts:check and prettier all pass locally.