Query parameters incorrectly appended with '?' instead of '&' for URLs containing existing params

[MPratley]

openapi-fetch version

0.13.4

Description

When using openapi-fetch to make requests to endpoints that already contain query parameters, additional query parameters are incorrectly appended using a question mark (?) instead of an ampersand (&). This results in malformed URLs that may not be properly processed by the server.

Reproduction

Example:

import { createClient } from 'openapi-fetch'

const client = createClient({
  baseUrl: 'https://api.example.com'
})

const response = await client.GET('/index.php?task=users.getUsers', {
  params: {
    query: {
      page: '1',
      limit: '10'
    }
  }
})

Expected result

Given the example above, When adding additional params like { page: 1, limit: 10 } should result in:

"/index.php?task=users.getUsers&page=1&limit=10"

But currently results in

"/index.php?task=users.getUsers?page=1&limit=10"  // Invalid URL format, has two '?'s

We're interfacing with a legacy php application, where a lot of our "endpoints" are just calls to an old MVC framework. As a result, a load of our endpoints are structured as query params that define the requested resource + query params that change pagination / what entities to return. See the reproduction for an example.

(I'm happy to open a pull request for this - but may need pointing in the direction of the code the currently handles query param appends)