Support loading ES Modules

[beerose]

Continuation of #283

[tianyingchun]

it seems that this change aim to native esm with .mjs

but while our pckage.json declaretype:module with source file extention .ts,.js, it will as esm also.

[beerose]

it seems that this change aim to native esm with .mjs

but while our pckage.json declaretype:module with source file extention .ts,.js, it will as esm also.

AFAIK that's something that typescript loader will take care of. @d-fischer can you confirm?

[tianyingchun]

it seems has a bug here

const loadJs: LoaderAsync = async function loadJs(filepath) {
  try {
    const { href } = pathToFileURL(filepath);
    return (await import(href)).default;
  } catch (error) {
    return loadJsSync(filepath, null);
  }
};

if we have named export here it will always return undefined

maybe it should be changed to

const { href } = pathToFileURL(filepath);
const moduleData =  await import(href);
return moduleData.default.|| moduleData;

consider below case

consider below example
`valid.config.js`

```ts
import { defineConfig } from 'vite';

// it is `esm` module causeof type:`module`
export const test = defineConfig({
  cake: 'a lie',
});

it will aways return me undefined.

@beerose 

[d-fischer]

I don't see why it should load a non-default export from the file. Do you have any actual use case for this? I'm pretty sure that vite does not use cosmiconfig.

Please don't crosspost your issues. Continued in #308.