refactor!: move KyselyTypegenPostgresDialect export to /postgres

To prepare for additional dialects and keep the default entry tiny,
`KyselyTypegenPostgresDialect` is now exported from a dedicated
`kysely-typegen/postgres` subpath instead of the package root.

The `KyselyTypegenDialect` base class is also reworked to support
dialects where enums are declared per-column rather
than as a named top-level type. The abstract `getEnumsMap()` method
is replaced by an optional `introspectEnums()` hook returning both
`named` and `inline` enums.

BREAKING CHANGE: `KyselyTypegenPostgresDialect` is no longer exported
from `kysely-typegen`. Import it from `kysely-typegen/postgres`.

Before:

```ts
import { KyselyTypegenPostgresDialect } from "kysely-typegen"
```

After:

```ts
import { KyselyTypegenPostgresDialect } from "kysely-typegen/postgres"
```

BREAKING CHANGE: custom dialects extending `KyselyTypegenDialect`
must replace `protected getEnumsMap()` with `protected introspectEnums()`,
which returns `{ named, inline }` instead of a flat `Map<string, string[]>`.

Before:

```ts
protected async getEnumsMap(): Promise<Map<string, string[]>> {
  return new Map([["Role", ["admin", "member"]]])
}
```

After:

```ts
protected override async introspectEnums(): Promise<IntrospectedEnums> {
  return {
    named: [{ name: "Role", values: ["admin", "member"] }],
    inline: new Map(),
  }
}
```

package.json

@@ -22,6 +22,7 @@
   "types": "./dist/index.d.mts",
   "exports": {
     ".": "./dist/index.mjs",
+    "./postgres": "./dist/postgres.mjs",
     "./package.json": "./package.json"
   },
   "publishConfig": {
@@ -40,11 +41,13 @@
   "devDependencies": {
     "@testcontainers/postgresql": "12.0.0",
     "@types/node": "25.9.1",
+    "@types/pg": "8.20.0",
     "kysely": "0.29.2",
     "kysely-postgres-js": "3.0.0",
     "oxfmt": "0.51.0",
     "oxlint": "1.66.0",
     "oxlint-tsgolint": "0.23.0",
+    "pg": "8.21.0",
     "postgres": "3.4.9",
     "semantic-release": "25.0.3",
     "tsdown": "0.22.0",