[BUG] [typescript-fetch] oneOf with primitive type gives compilation errors

Description

a oneOf schema that has a primitive type, such as string, as one of the options generates invalid typescript code that doesn't compile, (but can be fixed with a couple small manual tweaks)

(I think this also applies to anyOf)

openapi-generator version

typescript-fetch, using openapi-generator version 6.2.1

OpenAPI declaration file content or url

{
   "components": {
      "schemas": {
         "StringSchema": {
            "description": "meep ent",
            "type": "string"
         },
         "FooEntity": {
            "oneOf": [
               {
                  "$ref": "#/components/schemas/ObjectA"
               },
               {
                  "$ref": "#/components/schemas/StringSchema"
               }
            ]
         }
      }
   }
}

this generates code that looks like:

import {
  ObjectA,
  instanceOfObjectA,
  ObjectAFromJSON,
  ObjectAFromJSONTyped,
  ObjectAToJSON,
} from './ObjectA';
import {
  string,
  instanceOfstring,
  stringFromJSON,
  stringFromJSONTyped,
  stringToJSON,
} from './string'; // <-- THIS IS INVALID

/**
* @type FooEntity
* 
* @export
*/
export type FooEntity = ObjectA | string;

Also, the JSON-serialization function that gets generated (FooEntityToJSON) is invalid too.

Steps to reproduce

generate a typescript client using from the given example schemas above ☝️

Related issues/PRs

Suggest a fix

• primitive types in oneOf shouldn't have imports generated for them.
• the serialization function should use typeof(...) === "string" instead of instanceOfstring
• the serialization if checks for primtives should be moved to be before the ifs of objects

Start by reproducing the issue with the provided OpenAPI schemas using the typescript-fetch generator, then inspect the generated FooEntity imports and FooEntityToJSON function. Done means the generated TypeScript compiles, primitive oneOf handling avoids invalid imports and instanceOfstring checks, and the serialization branches work for both primitive and object options.