SelectedItemGroup: Type should be a generic

[jasny]

Describe the bug

While SelectItem is a generic, SelectedItemGroup is not.

Environment

N/A

Reproducer

No response

Angular version

18.2.8

PrimeNG version

17.18.11

Build / Runtime

TypeScript

Language

TypeScript

Node version (for AoT issues node --version)

n/a

Browser(s)

No response

Steps to reproduce the behavior

import { SelectItemGroup } from 'primeng/api';

const items: SelectItemGroup<string>;

Expected behavior

Type 'string' should be passed to type of items property.