[BUG][typescript(experimental)] Support Deno v1.6

Created by: chibat

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
[Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Generated Deno code does not work with Deno v1.6 .

openapi-generator version

v5.0.0

OpenAPI declaration file content or url

https://petstore.swagger.io/v2/swagger.json

Steps to reproduce

$ java -jar openapi-generator-cli-5.0.0.jar generate -i https://petstore.swagger.io/v2/swagger.json -g typescript --additional-properties=platform=deno -o generated-src

$ echo '
import * as petstore from "./generated-src/index.ts";

const configuration = petstore.createConfiguration();
const petApi = new petstore.PetApi(configuration);
const pets = await petApi.findPetsByStatus(["available"]);
console.log(pets.slice(0, 5));
' > test.ts

$ deno --version
deno 1.6.2 (release, x86_64-unknown-linux-gnu)
v8 8.8.278.2
typescript 4.1.3

$ deno run --allow-net=petstore.swagger.io test.ts
Check file:///home/gitpod/work/test.ts
error: TS1205 [ERROR]: Re-exporting a type when the '--isolatedModules' flag is provided requires using 'export type'.
export { createConfiguration, Configuration } from "./configuration.ts"
                              ~~~~~~~~~~~~~
    at file:///home/gitpod/work/generated-src/index.ts:4:31

TS1205 [ERROR]: Re-exporting a type when the '--isolatedModules' flag is provided requires using 'export type'.
export { PromiseMiddleware as Middleware } from './middleware.ts';
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    at file:///home/gitpod/work/generated-src/index.ts:8:10

Found 2 errors.

Generation Details

actual

export { createConfiguration, Configuration } from "./configuration.ts"
export { PromiseMiddleware as Middleware } from './middleware.ts';

expect

export { createConfiguration } from "./configuration.ts"
export type { Configuration } from "./configuration.ts"
export type { PromiseMiddleware as Middleware } from './middleware.ts';

Related issues/PRs

https://github.com/denoland/deno/pull/8482

Suggest a fix

I will create a PR.