[Pg]: Add PostgreSQL 18 OLD/NEW returning support

[fabric0de]

Summary

This PR adds support for PostgreSQL 18's RETURNING WITH (OLD AS ..., NEW AS ...) syntax to the PostgreSQL dialect.

It introduces an opt-in API for returning the previous and resulting row values from mutation queries:

await db
  .update(users)
  .set({ name: 'Jane' })
  .returning({
    old: true,
    new: true,
  });

The same API is supported for PostgreSQL INSERT, UPDATE, and DELETE queries.

Behavior

The returned values follow PostgreSQL's OLD and NEW semantics:

• INSERT: old is null, and new contains the inserted row.
• UPDATE: old contains the row before the update, and new contains the updated row.
• DELETE: old contains the deleted row, and new is null.
• INSERT ... ON CONFLICT DO UPDATE: the values reflect the outcome of each returned row.

Either value can be requested independently:

await db
  .delete(users)
  .where(eq(users.id, 1))
  .returning({ old: true });

The existing .returning() and .returning({ id: users.id }) APIs remain unchanged.

Implementation

The PostgreSQL dialect emits aliased OLD and NEW rows using:

RETURNING WITH (
  OLD AS "__drizzle_old",
  NEW AS "__drizzle_new"
)

The result mapper reconstructs the nested old and new objects and preserves null when a row version does not exist.

OLD/NEW projections cannot be mixed with regular returning fields. Invalid mixed projections are rejected at both the type and runtime levels.

This initial implementation intentionally supports whole-row old and new projections only. Partial OLD/NEW projections and other advanced projection forms are outside the scope of this PR.

Tests

The implementation includes:

• Type tests for INSERT, UPDATE, and DELETE
• Type-level rejection of mixed OLD/NEW and regular projections
• SQL generation tests
• PostgreSQL 18 integration tests executed in a dedicated CI shard
• INSERT ... ON CONFLICT DO UPDATE coverage
• Prepared-query coverage
• Driver-side result-mapping coverage
• Rows containing only nullable values

Related to #5109