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(),
  }
}
```

tsdown.config.ts

@@ -1,7 +1,7 @@
 import { defineConfig } from "tsdown"
 
 export default defineConfig({
-  entry: "./src/index.ts",
+  entry: ["./src/index.ts", "./src/postgres.ts"],
   dts: true,
   exports: true,
 })