feat: @payloadcms/db-vercel-postgres adapter (#7806)

Dedicated adapter for Vercel Postgres

- Uses the `@vercel/postgres` package under the hood.
- No `pg` dependency, speeds up invocation
- Includes refactoring all base postgres functionality into a
`BasePostgresAdapter` type, which will ease implementation of [other
adapters supported by
drizzle-orm](https://orm.drizzle.team/docs/get-started-postgresql)

## Usage

```ts
import { buildConfig } from 'payload'
import { vercelPostgresAdapter } from '@payloadcms/db-vercel-postgres'

export default buildConfig({
  db: vercelPostgresAdapter({
    pool: {
      connectionString: process.env.DATABASE_URI,
    },
  }),
  // ...rest of config
})
```

### Automatic Connection String Detection

Have Vercel automatically detect from environment variable (typically
`process.env.POSTGRES_URL`)

```ts
export default buildConfig({
  db: postgresAdapter(),
  // ...rest of config
})
```

test/package.json

@@ -28,6 +28,7 @@
     "@payloadcms/db-mongodb": "workspace:*",
     "@payloadcms/db-postgres": "workspace:*",
     "@payloadcms/db-sqlite": "workspace:*",
+    "@payloadcms/db-vercel-postgres": "workspace:*",
     "@payloadcms/drizzle": "workspace:*",
     "@payloadcms/email-nodemailer": "workspace:*",
     "@payloadcms/email-resend": "workspace:*",