[BUG][typescript-angular] generator generates invalid TypeScript for polymorphic types

Created by: t03apt

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

[typescript-angular] generator generates invalid TypeScript for polymorphic types. For example:

import { Circle } from '../model/models';
import { Rectangle } from '../model/models';
import { Rectangle | Circle } from '../model/models';

Expected:

import { Rectangle, Circle } from '../model/models';

openapi-generator version

4.2.3

How to reproduce

openapi-generator generate -i https://gist.githubusercontent.com/t03apt/71391e24045ef08dcad11bfae12037ae/raw/9c83bd41be70063d1436f84c842253018a6c8c46/Swashbuckle.AspNetCore.Test.WebSites.Basic.swagger.json -o ./generated -g typescript-angular --skip-validate-spec