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(),
  }
}
```

src/postgres.ts

@@ -0,0 +1,65 @@
+import type { EnumMetadata, IntrospectedEnums } from "./index.ts"
+import { KyselyTypegenDialect } from "./index.ts"
+
+export class KyselyTypegenPostgresDialect extends KyselyTypegenDialect {
+  // These types have been found through experimentation in Adminer and in the 'pg' source code.
+  public override readonly scalars: Record<string, string> = {
+    bit: "string",
+    bool: "boolean",
+    box: "string",
+    bpchar: "string",
+    bytea: "Buffer",
+    cidr: "string",
+    date: "Timestamp",
+    float4: "number",
+    float8: "number",
+    inet: "string",
+    int2: "number",
+    int4: "number",
+    int8: "Int8",
+    json: "Json",
+    jsonb: "Json",
+    line: "string",
+    lseg: "string",
+    macaddr: "string",
+    money: "string",
+    numeric: "Numeric",
+    oid: "number",
+    path: "string",
+    polygon: "string",
+    text: "string",
+    time: "string",
+    timestamp: "Timestamp",
+    timestamptz: "Timestamp",
+    timetz: "string",
+    tsquery: "string",
+    tsvector: "string",
+    uuid: "string",
+    varbit: "string",
+    varchar: "string",
+    xml: "string",
+  }
+
+  protected override async introspectEnums(): Promise<IntrospectedEnums> {
+    const rows = (await this.database
+      .withoutPlugins()
+      .selectFrom("pg_type as type")
+      .innerJoin("pg_enum as enum", "type.oid", "enum.enumtypid")
+      .select(["type.typname as name", "enum.enumlabel as value"])
+      .execute()) as Array<{ name: string; value: string }>
+    const grouped = new Map<string, string[]>()
+    for (const { name, value } of rows) {
+      const existing = grouped.get(name)
+      if (existing == null) {
+        grouped.set(name, [value])
+      } else {
+        existing.push(value)
+      }
+    }
+    const named: EnumMetadata[] = []
+    for (const [name, values] of grouped) {
+      named.push({ name, values })
+    }
+    return { named, inline: new Map() }
+  }
+}