Remove implicit reading of vitest.config.ts/vite.config.ts files

The implicit vitest.config.ts reading ends up being more confusing than it's worth. I think it would be much simpler to allow users to do this:

```ts
// evalite.config.ts
import { defineConfig } from 'evalite/config';
import config from './vite.config.ts';

export const defineConfig({
  viteConfig: config,
});
```

This would probably solve issues like: 

#95
#296