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
})
```

packages/db-vercel-postgres/eslint.config.js

@@ -0,0 +1,20 @@
+import { rootEslintConfig, rootParserOptions } from '../../eslint.config.js'
+
+/** @typedef {import('eslint').Linter.FlatConfig} */
+let FlatConfig
+
+/** @type {FlatConfig[]} */
+export const index = [
+  ...rootEslintConfig,
+  {
+    languageOptions: {
+      parserOptions: {
+        project: './tsconfig.json',
+        tsconfigDirName: import.meta.dirname,
+        ...rootParserOptions,
+      },
+    },
+  },
+]
+
+export default index