[Pg] Add MERGE statement support

[klotztech]

Adds MERGE statement support for PostgreSQL 15+ via a new db.merge() query builder, following the same fluent API style as UPDATE .. FROM and INSERT INTO .. SELECT.

Resolves #1298

API

// Upsert: update on match, insert otherwise
await db.merge(users)
  .using(newUsers, eq(users.id, newUsers.id))
  .whenMatched()
    .update({ name: newUsers.name, email: newUsers.email })
  .whenNotMatched()
    .insert({ id: newUsers.id, name: newUsers.name, email: newUsers.email });

// Multiple conditional clauses
await db.merge(users)
  .using(updates, eq(users.id, updates.id))
  .whenMatched(eq(updates.status, 'deleted'))
    .delete()
  .whenMatched()
    .update({ name: updates.name })
  .whenNotMatched()
    .doNothing();

// RETURNING (requires PostgreSQL 17+)
const result = await db.merge(users)
  .using(newUsers, eq(users.id, newUsers.id))
  .whenMatched()
    .update({ name: newUsers.name })
  .returning();

// Subquery as source
const filtered = qb.select({ id: src.id, name: src.name })
  .from(src)
  .where(isNotNull(src.name))
  .as('filtered_source');

await db.merge(users)
  .using(filtered, eq(users.id, filtered.id))
  .whenMatched()
    .update({ name: filtered.name })
  .whenNotMatched()
    .insert({ id: filtered.id, name: filtered.name });

// Works with .with() CTEs
await db.with(cte).merge(users)
  .using(cte, eq(users.id, cte.id))
  .whenMatched()
    .update({ name: cte.name });

What was changed

• drizzle-orm/src/pg-core/query-builders/merge.ts — New file. Implements PgMergeBuilder (returned by db.merge()), PgMergeBase (the main builder after .using()), PgMergeMatchedActionBuilder (.whenMatched() actions), and PgMergeNotMatchedActionBuilder (.whenNotMatched() actions). Supports .returning() for PostgreSQL 17+, .toSQL(), .prepare(), and CTE integration via db.with().

• drizzle-orm/src/pg-core/dialect.ts — Added buildMergeQuery() to PgDialect. Generates the full MERGE INTO … USING … ON … WHEN … THEN … SQL, including conditional predicates on each clause and an optional RETURNING clause.

• drizzle-orm/src/pg-core/query-builders/index.ts — Re-exports all public types and classes from merge.ts.

• drizzle-orm/src/pg-core/db.ts — Added db.merge(table) to PgDatabase and to the db.with(…) builder chain.

• drizzle-orm/tests/merge.test.ts — 16 unit tests covering all clause types (.update(), .delete(), .doNothing(), .insert()), conditional predicates, literal params, column references, SQL expressions, RETURNING, and subquery sources.

• drizzle-orm/type-tests/pg/merge.ts — Compile-time type tests verifying the return types of all builder combinations.