[BUG][angular-typescript] Generated model property names not compatbile with api spec in case of reserved keywords

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 model classes have wrong property names in case they are reserved keywords. I guess the same would happen if they are not
valid typescript identifiers for other reasons.

openapi-generator version

all versions I know. Tested on 4.3.1 and master.

OpenAPI declaration file content or url

api.yaml:

openapi: '3.0.0'
info:
  description: Reproducer for openapi issue
  version: 1.0.0
  title: Reproducer for openapi issue
servers:
  - url: https://example.com
paths:
  /inputOptions:
    get:
      responses:
        200:
          description: An api call returning the ExampleObject with broken model
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ExampleObject'
components:
  schemas:
    ExampleObject:
      type: object
      required: ['function']
      properties:
        function:
          type: string

Command line used for generation

java -jar ~/git/openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i api.yaml -o generated/ -g typescript-angular

Steps to reproduce

Using the generated client with an backend using the original property name - in my case a spring-java backend will cause the _function property to be undefined while the function property is set (but not declared in the generated model). This is not very surprising, so looking at the generated file generated/model/exampleObject.ts should be enough:

export interface ExampleObject { 
    _function: string;
}

Related issues/PRs

Suggest a fix

The generated code should use the original property names to be compatible with the api spec. If names cannot be used safely as identifiers, StringLiterals could be used (see https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#22-names):

export interface ExampleObject { 
    "function": string;
}

Start with the api.yaml reproducer and the typescript-angular generator command, then inspect generated/model/exampleObject.ts. Compare the generated _function property with the original function name and the suggested TypeScript string-literal form. Done means regenerating the model preserves the API property name safely.